To answer your question:

Some are prepaid, some are not.
Some are only prepaid valid in certain countries, others worldwide.

Also, some shops don't even give you an envelope, so if you are unable (or elect not) to process your refund form with a reclaim company/agent desk at the airport (after customs stamping), then you have to post it back at your own expense of an envelope too!

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To be pedantic, the reply envelopes have nothing to do with customs.

The reclaim forms (the contents of the envelopes) should be stamped by customs before you are using the evelope. And the envelope is used to post the stamped form back to the retailer - or more likely the outsourced reclaim company the retailer is using.

[The only time you would use the evelope with customs would be in the rare case where customs have (via a courtesy phone, etc) requested that you leave your form in a box for them to stamp later - and they will post it to your refund company after stamping.

In this case, don't seal the envelope (obviously). Also it is for this reason that I would advise anyone who has a form but was not given an envelope to carry their own. While you might think "It's ok, if I can't get it processed there and then by the refund company after stamping, I'll post it back from home - I have envelopes at home", that is no use if customs ask you to leave the form with them.]