A brief note explaining that only 2 months are available to you. Surely you have a lot more than the equivalent of 20,000 baht? They are more interested in you showing financial stability than 3 months of statements.
Why don't you state the exact embassy you are planning to apply at, for a better than generic answer. And exactly which documents are you talking about?
Was the statement and closing certificate in a single .pdf file? You can only upload 1 file per section of the application form. Maybe resend the statement. I think the "other statement mentioned is for a sponsorship arrangement."