I think it's enough. Although I don't live in the US, I always send 2-3 IRCs when writing overseas and there has been no problem with it.

Many celebrities do cover the rest of the postage if it isn't enough to send back as SelenaandDemiFan has said, but who knows? You'll have to see.

It depends on the weight of your package.

However, if it's not enough, most celebrities should cover the rest.

If you've already sent out, than it's a moot question. You'll just have to wait and see.

If you haven't sent out already, then why waste your money on IRCs? There are some nice folks on this site who can get UK stamps to you. I don't mean to sound harsh or mean, but you've been on this site just a little longer than I have and you should know by now the resources available to you here.

is two irc enough for the letter coming back from the uk to seattle. that is how much i put in there was two irc.