Special Payment terms

Hello my friends.

I want to consult you during a new R12 Implementation project.

my customer uses various payment terms that he gives his customers.

part of the payment terms he needs cannot be setup by the standard functionality of oracle.

for example:

1) End of week + X days

2) 2 permanent payment days in a month (if invoice date falls between 1-14 of the month- payment term is 15,

if invoice date falls between 15-31 of the month- payment term is 1 of next month).

We will use order management + manual invoices.

Does anybody have an idea how can I customize the transactions to have the correct due date according to special payment terms like these?