help me any suggestion: wanna know the exact business need invoices

hay there!
i am a student in the scondary school "17m"i recently posted on the reddit about any suggestion to give me idea about saas product and sombody told me to do some small stuff for people and businesses and get paid for it and then make some money to start working on making the saas product he suggest me to do work small stuff like invoice etc so now i want you to do the same what he suggest me to do so i want to get the suggestion from you guys to tell me what are the business could be a local business as well as digital buinsess who iterate with the invoice every single month and desperatly wanna to automate the work so i want to make the automation for them of the invoice work and get paid for so just tell me what are the business who want to automate the invoice work so i can target then and wish from the god to make some little money with it
i only just want your suggestion from your where you feel this or that business have a huge work of invoices.