Background Check- HUGE problem

So I'm currently going through the background check process for a BB for an SA position. Apparently this BB does a pre-screening themselves first and then uses a background check agency for extra checks.

I'm currently in the first stage (pre-screening done by their internal team). As far as I know everything checks out, but one thing: on the online forms I had to fill out, I mentioned that I had, 4 years ago, a department store credit card account that went past due (for a total amount of like 200 dollars) that I ultimately setted with a third party. In hindsight, I probably should not have even mentioned it, as it was such an insignificant event. However, I did so for the sake of full disclosure.

Now, they want documentation proving that this account was indeed settled. Now, this happened 4 years ago...and I no longer have the letter from the credit collection agency (if I ever even received one). So, I ran a credit report on myself and sent it to HR, proving that there were no outstanding judgements against me, that I had no current delinquent accounts, and that my credit score was very high. Furthermore, the report showed my past due account being closed and purchased by a third party and also showed the subsequent account with the third party being legally settled.

Somehow, this is not being considered adequate documentation, and they want a letter of proof that this thing was settled. I imagine this is going to be quite difficult to obtain as i)this is dealing with a credit collector and ii) i am currently overseas. I fear that I may not be able to provide HR with this document.

Does anyone:
1) have any tips with how to deal with HR concerning this
2) know if my offer is in jeopardy due to this? - I've been 100% transparent, honest -no misrepresentation and complete cooperation