Do you keep envelopes in inventory????
Anyway, receive the envelopes as you would any other item, the method of payment is irrelevant at this point. There is no facility to process cash purchases per se.
Assuming you are using AP, IC & PO you would:
1. Receive items in PO: PO Transactions, Receipt Entry - you can enter the invoice number at this point as well.
2. Run IC day end processing.
3. Post AP invoice batch list.
4. Enter cash payment against the AP invoice.
You could enter the cash payment in AP as a prepayment on the vendor, this will auto apply if you use the correct invoice or PO number.