Freshbooks integration with Membership

Since we Dutch people (well, businesses) have to make invoices when we charge people for anything I can't use Membership out-of-the-box (not without making the invoices by hand, which - ofcourse - I'd rather not do).
I am thinking of integrating it with Freshbooks, I figured this would give me the least amount of work.

Are there any things I should keep in mind when making such an integration?