A point of sale system built around invoices and currencies.

The brief: connect the counter to the customer account

The project began with a request for an original point of sale platform. Invoicing became a core requirement immediately: the system needed to handle services and customer accounts alongside ordinary counter sales.

The build had to keep a quotation, invoice, payment, credit and stock movement connected. Creating another document for a sale must not create another sale. Supplying the same order twice by mistake must not remove stock twice.

Currency handling was the next major requirement. The owner wanted different transaction currencies within one business workspace and exchange rates collected automatically from a named source.

A register with a persistent basket

The register brings product lookup and the current basket into the same workspace. An operator can scan or search for an item, change its quantity, apply a permitted discount and attach a customer before recording payment.

A held sale can be resumed later. The device retains its basket across a page reload. The current release requires a connection to save the completed sale and business records.

The interface also provides access to invoices, quotations, catalogue items, stock, customers, suppliers, purchase orders, register sessions and reports. These records use the same business context.

Invoices that retain the payment history

A quotation can become a draft invoice without retyping its lines. On issue, the invoice receives its final number and retains its customer, prices, currency and terms. Payments reduce its outstanding balance as they are recorded.

Deposits and later payments remain separate entries against the original invoice. Credit notes reduce what is owed, and refund records show money returned. Customer statements bring those events into one account history.

The invoice and fulfilment records remain linked. The stock deduction happens through fulfilment once. A second completion request returns the existing result and leaves the stock movement unchanged.

For recurring work, a schedule produces draft invoices for review. Weekly, monthly, quarterly and yearly schedules are available. The operator issues the draft before sharing it.

24 currencies in one business workspace

Each invoice has a transaction currency. The business also has a reporting currency, used to combine its management totals. The original transaction amount remains visible alongside its reporting equivalent.

The supported currencies include TTD, USD, GBP, EUR, CAD, BBD, JMD, XCD and GYD. The calculation layer also handles currencies with zero or three decimal places. Customer preferences help select the currency on a new document.

Cash registers retain separate native balances. At closing, the operator counts each currency held. This makes it possible to identify a USD cash difference without burying it in a converted TTD total.

From a USD quotation to a TTD report

Consider a service quoted at USD 250. At an illustrative reference rate of 1 USD = 6.80 TTD, the invoice has a TTD 1,700 reporting value. The quotation and invoice show the agreed USD amount. This is a worked example, not a customer sale or a current exchange rate.

A confirmed USD 100 deposit leaves USD 150 to collect. A later USD 150 payment settles the same invoice. The reporting values use the rate retained with the issued document, and the payment history keeps both references.

If a physical product is part of the order, its stock movement follows fulfilment. Reprinting the invoice or repeating a completed request leaves the quantity unchanged. The customer account, register and stock report can then be checked against the same transaction.

Automatic rates with a transaction history

The system collects daily reference rates from ExchangeRate-API. An hourly check follows the provider publication schedule, and the settings page shows the source, publication time, last successful sync and currency coverage.

Each issued foreign currency invoice retains the rate used for reporting. Later updates do not change the customer price or rewrite the issued invoice. A source failure leaves the last validated data available, with its age visible.

The feed checks reject invalid values, incomplete currency sets and older publications. Rates more than 48 hours old cannot be used for new automatic foreign currency invoices. Existing invoices retain their records during an outage.

Reference rates support pricing and reporting. Actual bank conversion, processor fees and settlement differences require their own payment records.

ExchangeRate-API data methodology / ExchangeRate-API publication schedule

Stock movements and cash closing

Products have catalogue records, stock quantities and movement history. Receiving, counts and location transfers record how the physical quantity changes. Supplier and purchase order records support the purchasing workflow.

A register session starts with an opening float. Cash paid in, cash paid out, refunds and sales contribute to the expected closing balance. The operator records the count and explains a difference before closing.

Reports show sales, payment records, customer balances and stock information. Exports preserve the transaction currency and recorded conversion details so the figures can be followed outside the application.

Testing the transactions that can go wrong

The transaction tests covered discounts and tax rounding, deposits, excess payments, credit quantities, stock shortages, repeated requests and account boundaries. Failed stock operations had to leave no partial transaction behind.

Currency tests included whole unit and three decimal currencies, partial payments, quotations converted to invoices and cash counts in more than one currency. Feed tests covered delayed updates, invalid data and changes after an invoice had been issued.

Browser checks followed the invoice and checkout workflows on desktop and phone sized layouts. The images in this case study use sample records from those checks.

What is in the current release

DSDillon POS is running on version 1.2.0. It includes checkout, quotations, invoicing, customer balances, stock records, register reconciliation, 24 currencies and automatic exchange rates.

An operator confirms each external payment before recording it. Live Stripe collection, direct invoice emails, completed offline cash sales and the wider premium modules are under development.

Existing account holders can sign in through the POS link on this page. A business implementation starts with its invoice requirements, devices, payment providers, opening stock and customer balances. These form the acceptance tests for the deployment.