What do I need to include in confirmation letters for my DTV visa application as a freelancer in Vientiane?

I have my appointment for DTV application in Vientiane, in 2 weeks. Getting my supporting documents together and have some questions someone might be able to help with.

FYI, I'm applying as a freelancer.

1, What information should be included in the "confirmation" letters from clients?

2, Do these letters just need to have client's company letterhead/logo and be signed and dated by them? Or will they also need to include their company stamp too?

3, Can I pay the visa fee in Thai baht at the consulate on the day?

Any insights from successful applicants would be great.

Thanks