What is the difference between Letter size and A4 size? Why are you offering both?

Printing

Letter size paper is used mainly in North America while most of the rest of the world uses A4.

A4 size: 210 x 297 mm / 8.26 x 11.7 inches
Letter size: 216 x 279 mm / 8.5 x 11 inches

I offer both because I personally use A4 size, but a lot of my customers are in the US, so it makes sense to include both sizes.