Switch on the customer ledger
Show a running 'previous balance + this invoice' on every receipt.
Open Ledger Activation
Go to Get Paid → Ledger Activation.
Backfill your history
Run Backfill (do the dry-run preview first, then the real run) to bring your past transactions into the new running ledger, then Run Verification to check the numbers match.
Activate
Tap Activate to turn on the cumulative balance so customers see their running total on every receipt.