If you do any of the higher value offers I strongly recommend taking screenshots and saving any confirmation emails, etc. That way you have a better chance to "prove" that you completed the offer. Also be very careful to complete every criteria in the offer.
I have done 2 offers and in both cases I didn't get any credits. The support however was very helpful and manually transferred the credits I was entitled to (after providing screenshots and confirmation emails as proof).
Also be patient, it can take a day or two to get the credits, same goes for replies from support.