I've ordered some broken and missing pieces twice and I think it is very easy. I mailed to Playmobil by using the Contact button on the website, once and they sent me a form through mail. I've printed this and filled out the article numbers and sent it by post to their office in Belgium (Mechelen) You allow them to do a automatic payment for a maximum amount. Within two weeks they send the things you ordered (if still available).
Shipping costs are around 5 euro's if I remember correctly.
I'm afraid you'll get hooked. Any part of a new set you want is within reach now...
I'll soon order some of the new animals from big sets that I don't have yet.