Payment module problem

[nilla]

I have had a bookshop in OsCommerce for many years with two paymewnt options: invoice and payment in advance. It has worked fine. Since a couple of months I have added Pay with Payson but it didn´t work and I have removed the Payson option. My problem is that the option payment with invoice (together with the product) has disappeared from the shop although it still is chosen together with payment in advance in the admin payment site and when I try to add it once again I can´t find a choice that suits the option when I send the invoice together with the product. How do I do to get this option back again?