To access this feature, you must be on Goodshuffle Pro Standard or above.
If you record the Offline payment in the billing tab:
Go to contract and click "send to client" (I'd just change the word "invoice" in the subject and email to "receipt").
It'll send them the PDF of the invoice with the payment record and a link to their live invoice which will show their payment reflected.
Or, you can email them the Payment Record document from the Messages tab:
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