I’m trying to file my taxes online (here: 軟體下載與報稅：財政部電子申報繳稅服務網). I am self-employed, and it says I should report my income and expenses in a supplementary document. I don’t see a way to add that though. Should I mail it separately somewhere? I see a way to import a .dat file, but it doesn’t specify what kind of format that should be.
Also, is it sufficient proof to just show my US tax return, or do I need receipts for everything?
Thanks.
It generates some printouts on one of the final screens telling you what they want you to submit, and maybe an address label as well. Or you can just take them to the tax office in person.
The Kaohsiung Tax Bureau published an e-mail for foreigners to send their proof of overseas income to: foreigner_service@ntbk.gov.tw. Maybe for other cities/counties the e-mail address is the same, just with a different tag (like the “k” for Kaohsiung).
I have a remote employment so I just collected together 12 months of payslips in one PDF. I asked this question before and I was told that it would suffice. If your situation is more complicated, I am not sure how you should do it, but as a rule of thumb I would say evidence of the figures that put together make up for the income that you declared, i.e. payslips, invoices, etc.
What Andrew said, print and post to them
What happens if they have any questions or disagree with some of the information and adjust the tax payable because of that? Would that incur any penalty?
Was planning to go in person, but maybe I could first try the online filing and see what happens 
There was a technical hiccup during my online filing, so I had to go in person to complete the process. My impression is that none of them would take the responsibility to tell you whether your documents are valid, invalid, sufficient, insufficient, what else you should provide, etc. but only give you an e-mail address to contact as they did with me. They are just there to robotically help you file (on the same online system that you would use at home btw) with whatever data you give them, especially when it’s overseas income.