Not sure where you are but in the US, you don’t need any business license for many types of businesses especially when freelancing. And you can invoice using any method including handwritten.

It is probably smarter to do something better than handwritten invoice but I am glad there are not so many hurdles to starting a business here. I have dealt with a lot of contractors here who don’t even issue a paper invoice for small jobs, they just tell me how much I owe and I just pay.

Small jobs probably are not worth suing for. But I’m sure if I don’t pay for a big enough job, they would sue me.