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So on one of the posts about mailing it says it cost $3.76 to post a large letter from USA to UK as long as it's no more than 100g, is a large letter equal to A4?

Also how much postage would I need for a letter half the size A5 from USA to UK again no more than 100g.

Last questions is how many of the forever stamps would that = to both the A4 and A5 envelope? It's so complicated, I just want to make sure I am putting enough postage on!

100g is roughly 3.53oz.

A4 counts as large envelope size. This will indeed cost $3.76 to send from US to UK.

A5 and under would be considered regular letter size. This would cost $3.50. However, 3.5oz is the limit for standard letters, and if you pass this point, it switches to large envelope prices.

Forever stamps are currently worth 44c each, so you would need 9 to cover the A4, or 8 to cover A5.

Thank you so much for your help (: