Snap punches are very hard to deal with however:
(i) you should move off centre line as soon as attack is imminent. Reading the aggression is one aspect of aikido. Also, you don't need to deal with the arm that is doing the snap punch to do aikido, and it is a fallacy that you cannot initiate the attack in aikido.
(ii) rokyu is a good one against snap punches. Other possibilities are irimi-nage (where you don't necessarily need the arm) and ude-garami.
(iii) you should be able to take a few snap punches; however an atemi of your own should induce a block of some type which you can take advantage of.
(iv) remember that 80% (give or take) of aikido is atemis!