I've booked 2 trips over the weekend and no problem.
I did discover that not all origination points offer holidays, but that wasn't the question.
In order for you to offered a installment facility, the holiday has to consist of 2 elements minimum.
This can be:
1. RT flights
2. A hotel or car booking for the entire duration of your stay.
3. However if your hotel or car reservation is less than the entire duration of your stay, you will require a 3rd element. So for some of my ex eu's I add a cheap hotel the night before departure.
Then the website should price it and offer installments.