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Centsless is designed for POS vendors who want to add multi-state cash rounding compliance for all their merchants with a single integration. You authenticate once with a partner API key, provision your merchants through that key, and then submit transactions on their behalf — the engine resolves the correct jurisdiction and rounding rule automatically for every cash sale.

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Integration overview

1

Get a Partner API key

Contact Centsless to obtain a partner API key. Partner keys carry the prefix centsless_partner_ and grant vendor-scoped access for merchant onboarding and transaction processing across your platform.
2

Provision each merchant

Before processing transactions, register each merchant on the Centsless platform using your partner key. Include a merchant_id that matches your internal merchant identifier — you’ll send this field with every transaction to associate it with the correct merchant record.
3

Process transactions

Submit each cash transaction to POST /transaction with merchant_id, the amounts (subtotal_cents and tax_total_cents), payment_method, and either zip_code (recommended) or state for jurisdiction resolution. The engine applies the correct rounding rule and returns the rounded total in real time.
4

Display the rounded amount

Surface cash_total_cents from the response on your cashier-facing and customer-facing displays. This is the legally correct amount for the customer to pay.
5

Store the transaction ID

Persist the transaction id returned in the response. You’ll need it to generate compliance receipts and to submit any subsequent overrides or voids against this transaction.

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Minimal transaction request

The four required fields are merchant_id, subtotal_cents, tax_total_cents, and payment_method. Adding zip_code enables automatic jurisdiction resolution. 
curl -X POST https://api.centsless.org/api/v1/transaction \
  -H "Content-Type: application/json" \
  -H "x-api-key: YOUR_API_KEY" \
  -d '{
    "merchant_id": "MERCHANT-001",
    "subtotal_cents": 1299,
    "tax_total_cents": 104,
    "payment_method": "cash",
    "zip_code": "85001"
  }'

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Response fields to surface in your POS UI

The transaction response contains several fields your integration should act on:
FieldWhere to surface it
cash_total_centsAmount to collect from the customer — show on cashier and customer displays
rounding_delta_centsDisplay on the receipt as a rounding adjustment line item
cash_rounding_modeLog for compliance recordkeeping
compliance_statusAlert the cashier or manager if the value is WARN or FAIL
fraud_score / risk_levelFlag for manager review if the risk level is elevated
Always pass zip_code for automatic jurisdiction resolution. The engine resolves state, county, and city from the ZIP and applies the correct rounding rule for that location. Only fall back to state, county, or city fields if you don’t have ZIP data for a merchant location.
Pass terminal_id and cashier_id on every transaction request. These optional fields are stored with the transaction record and provide employee-level audit attribution, which is essential for compliance investigations and override reporting.