Another question about invitation stamps

I have been doing them little by little and finally I just have 5 left!

But I have one question, two of my invitations are going to London. What should I do about the response envelopes? Am I correct in thinking that I can't put American stamps on those? Would it be alright then for me not to put return stamps on them?

Thanks in advance!

I found this on the knot:

clicky!

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Posted by Future Pepper

I found this on the knot:

clicky!

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Perfect! Thank you!