Hi Marco,
I suppose the first few lines are the same question in Dutch, so I’ll answer the English one. I would say that it depends on the way the interface is working. That’s probably an answer you don’t expect on this forum, but since you say that it is not possible with the Nesma guidelines, I assume that these connections are highly technical.
The FP answer is that you count the functionality the connections represent, usually one or more EO. When the interfaces contain highly complex messages the number of EO can be quite large. In our webshop we have XML connections that contain up to twenty EO per interface, so it could be an option to have a close look at the real functionality that is in these connections and then evaluate whether it makes sense to use function points or not.
When you are talking about highly technical connections you could agree with your supplier on a price per connection. We usually do a mix: a fixed price for the technical set-up and a variable price per FP for the functionality of the interface.
J'espère que cela t'aides.