No, I don't think you are expecting too much to expect a couple of extra eggs, but if the seller didn't specify that extras would be included, I wouldn't be upset, either. I feel that it is common courtesy to throw in a few extras, provided that the seller's hens go along with the plan.
I of course know and understand that sometimes the girls are fickle and extras aren't always available. I think that, as a seller, if you aren't sure if you'll have extras a disclaimer of sorts should be applied to the ad (e.g., extras will be included IF the hens provide them). If someone advertises "quantity+ extras," I like to get 2 or 3 extras. I always appreciate a few extra eggs to help my chances of getting a decent number of live chicks from the hatch.
Me personally, when I sell eggs, I won't sell a specific quantity (say 12, for example) unless I'm reasonably sure that my hens will lay that quantity PLUS several more within 3 days, 4 tops. So I don't like to sell 12 eggs without actually shipping at least 15, preferably somewhere between 16-18. I like to send plenty of extras.
Shipping eggs is such a gamble, I want to help the odds of the seller getting live chicks any way I can.