i'd start with one design first. makes tech support easier. plan to make 50-100 kits at first. unless its something awful you should be able to sell 100 of anything
as you work out the bugs you can stock more. you'll probably get a couple things wrong, especially with pricing/shipping.
for payment, go with paypal, i started with 'buy now buttons' and its not pretty but is very very easy. i think they may even have integrated USPS shipping now. priority mail, for example, is $5 for up to a pound, domestic with 'delivery confirmation'. that allows a lot of leeway. you can ship online for free ("click-n-ship" at usps.gov) & get free USPS carrier pickup.
once you have 3 solid designs you can move to an integrated cart. they take a -lot- of effort tho, especially if you go with an open source one.