I think that it is quite clear that having only a drink does not constitute dining, but there is no mention that one has to have a full multi course meal. Why not just order whatever it is you want to eat? GP will not deny you the 1,000 points anyway.

I agree that the certificate probably won`t post automatically- they never used to for me. It might be a good idea to make a photocopy of the certificate that you can fax in to CS if necessary.