I sell digital products which means that when you buy from my store, the products become available to you in matter of minutes, and around the world! So to answer your question: Yes, i do sell to US!

yes. you can scale up and down the size of the images without effecting the quality. what you should keep in mind however, is that the original size does have something to say about the details, texture and so on. therefore I recomand that you follow this: Example: Original size: A6 – Max scale up: A4.
Some artworks have enough details to scale it up to a A3 or bigger format, while others do better as A4. This also changes from person to person.
The original size of the artwork stands in the product description of the different artworks.

My print-guide for personal use:
- Original size: A6 – Max scale up: A4.
- Original size: A5 – Max scale up: A3.
- Original size: A4 – Max scale up: A2.
- Original size: A3 – Max scale up: A1.

There are different processes depending on the size and media. If the finished product in around A4 paper size, I scan the artwork using high quality and large dpi - usually between 500-1200 dpi.
These files are then used to create the posters made in photo editing programs or using canva. When preping for sale, I try to get as high resolution as Etsy allows me to upload, due to the files being large and etsy does have a limit of how large the files can be.
For my physical products, I use Gelato where I use as high resolution images as possible to create the prints, covers and other products with the best quality that I can.

If the size is larger than A4, I use camera with high resolution.

I use gelato, a company that produces different products with my prints on. If you are ordering these products, will the order go straight to them and in production in matter of 0-5 days. Then it is shipped directly to your address.

The stamps for Procreate are sketches of humans anatomy with different poses. If you use these to create your artwork, the art would still be your craft but with guide from my stamps. So if you are to use the stamps for commercial use: you can. But remember to credit me down in the descriptions if you used the stamps.