There is no dedicated feature for this in Flexi; however, the following approach can be used:
Selling vouchers
These vouchers must be registered in the price list (as gift vouchers)
Once these items are added to an invoice, the invoice should be sent to EET without any issues
Sales are entered into the invoice with a positive value
2) Redeeming a voucher by a customer
First, add the full invoice amount to the invoice (it can contain one or more line items, which may exceed the voucher amount)
Then add the voucher (price list item) to the invoice with a quantity of -1 (the invoice will show the total amount minus the voucher amount)
A stock issue document with a negative value will be created for the redeemed voucher (the voucher will be returned to stock)