Maybe this will help somewhat. I only say this because I originally had an O-A and am now on extensions. I get emailed to me in Bangkok a signed letter from my former employer, listing my company pension.
I have two other pensions. The Embassy told me I only need to do screen caps of my CPP and OAS pensions from my government account. Actually, I can print each formatted statement off from the screen or throw them onto a USB drive and print everything off in colour at a computer shop.
I take these to the Embassy. I remember filling out a form. They use a pre-formatted statement. A counsellor fills in the blanks, adds them up and issues the signed and stamped Statement of Income, addressed to Thai immigration, along the lines of To Whom It May Concern, which you present to Thai immigration. 1500 baht, I believe, which can change based on the exchange rate.
I also remember, the embassy changed to cashless payments, either bank draft or credit card. Fees listed on link below. Scroll down to Notarial Services, Payment Methods, Letter of Income:
https://www.international.gc.ca/country-pays/thailand-thailande/bangkok-info.aspx?lang=eng#NS