At first thought, it sounds a bit suspicious. However, I have seen many a people, do many a crazy thing. Maybe their in dire need of those seeds and are willing to pay the piper to get them fast at an added charge.
I myself have ordered many times and paid the overnight charge, just because I wanted it then and there, but never for something that was less than the actual cost of the item.
If you specified your terms prior to the sale, and they want to go against those terms, I would say go with your gut instinct, and stick by those, and go from there.
The gut is generally on the money. Jezt my .02
