Glad that you guys honored for all the people that spent on it, I would say that's the bare minimum, since that was what was advertised there.
In the other hand, this is very unfair to everyone else that were planning to buy and were busy/at work/etc. Everyone should be able to buy and be rewarded for the same amount of cookies.
If the event will be running for 3 weeks, the correct approach would be either:
A - Change it, so the reward is 11 instead of 1 for the entirety of the event;
B - For a fraction of the duration of the event (a few days / a week) give the advertised amount.
This would give EVERYONE and not only a handful of ppl to buy the advertised amount, if they want to do so.