For once off items that need to be due COD you can do the following:

When generating an invoice for a once off item/service in SnapBill (Billing -> Create an invoice) simply click the 'Change settings' link at the top of the Invoice Editor window.

Within the change settings area you can edit once off settings for the invoice you are about to generate. You can set custom once off due date settings here. Simply fill in the desired due date and generate the invoice by Saving as an Invoice.

These settings are not global and will only be applied to that specific once off invoice allowing you to have a custom due date.