Sorry to put a damper on things, but you generally need to be aware of the laws of the applicable to doing business with your customers wherever you are and they are.
For example, if you sell things to people in the state of Texas, the state of Texas will expect you to collect sales and use tax and remit that to the state government.
If you do business ...
I Wish this platform had a way to share documents/excels via source code, but here is an image example of what I use as a universal bill.
Type/format the following in a rich text document Document and you should be ready to go.
I've been using this format to receive payments from all across the globe including the US, EU nations, and various Asian ...