Hi there,
You should try to go to the postoffice in your homecountry (Sweden right?) and ask for International Reply Coupons (also called IRC on the forum). Not all countries sell them so your should ask first. If they do sell those you can include two IRC inside a SAE for the celeb to use to pay for the postage needed to send your letter back to you after replying.
If they don't sell IRC's you can add a couple of dollars (or pounds if your sending to the UK) inside the SAE instead. Maybe something like 2-3 dollars or pounds?
Celebs can use those to pay for the return postage instead
If you can't get any forgein money, just send a SAE and hope the SAE will be enough for you to get a reply.
As for the amount of postage you need to put on a envelope to send abroad from Sweden: that's different for every country so you should best ask the people at the postoffice
They will also know if your letter will be airmailed or differently send out:)
Good luck!