They all offer discreet shipping so no worries there, and they give you a choice to pay a bit extra for guaranteed delivery. If your package is nabbed by Customs, they'll ship another order to you.
As far as being affordable, I think that depends more on the brand of seeds you buy rather than the distributor. You can buy single seeds from $3 on up to $20+, depending on producer and strain. Some of the banks have bargain listings, I know Herbie's does. I have only used two different companies and only one of those still ships to the US, so that leaves Midweek Song as the one I can recommend. Reasonable prices, VERY fast delivery (six days to West Coast), huge supplier list, free seeds that you choose yourself from a selection they offer and they give a 20% discount on your order if you enter code "roll20" at checkout.