A lot of people from the US seem to forget that they're excluding the majority of customers by asking you to enter a zipcode that will only work in the US. It's incredibly annoying if you're part of that majority. With that said - good luck :-)
Being from outside US, I sympathize with that but: 1) It's a new service. It's reasonable to restrict your audience at first. Making it more international requires more work (shipping, billing, etc) which might end up being useless. It makes sense to test with domestic users first. 2) You should take a lot of care before shipping seeds internationally. Some countries (well, all countries I have flown to) have strict…
point 2) perhaps finding a local partner to ship stuff that is already in the country might be a decent way around this..