In my experience, I just offer immediately after the first is swallowed and they always take it. They are still in feed mode and I've never had a refusal.

If you have the finances and freezer space available then buying online is much, MUCH cheaper in the long run. The size comes fairly varied within a package. I sort them in ziplock bags of 3-5 depending on size and weight and then put those smaller bags into a ziplock gallon freezer bag. Wrap that in some walmart/grocery bags and they last months. Freezing a few in smaller bags keeps most airtight while opening only a few at a time.