Sending LOR to AACOMAS

This is probably a dumb question, but what is the best way to upload a LOR to AACOMAS? Should I use a 3rd party like Interfolio or send a link through the AACOMAS portal directly?

Are there any downsides of uploading directly to AACOMAS? I know interfolio charges, and correct me if I'm wrong, but AACOMAS is free?

Is the earliest I can send requests for letters through AACOMAS when it opens in May? I have a professsor who wants to upload a letter by next week.