Comingled Mail Data Merge

I'm trying to use Indesign in combination with BCC's Mail Manger to do a comingled varible data letter mailing that's been presorted for the post office. I have a cleint who's trying to mail to a list of around 4,000 recipents. However each one of those recipents will recieve 1 of 15 different letters (there a field in the spread sheet that says which letter they should get). The letter are donnation letters and in the actual body text of each of the 15 letters there is a varible data tag for the amount of money they're asking from each person (ie $50, $75, $1,200, etc...).

Now the fun part. The customer wants us to do all 15 letters to all 4,000 recipents as a single stream mail merge. If it weren't for the Varible Data in the body I would just save the body text of the 15 letters to PDF and have InDesign's data merge switch between them. To make matters worse the variable data tag is in a different place in each of the letters. So it's not like I could nest a text box within a text box and do it that way.

I'm wracking my brain for a way to do this digitallly, but so far the simplest solution I can come up with is to do 15 separate merges and then hand sort it into the correct order for mailing. I've attached two of the letters as examples. What will be varrible data is highlighted in yellow.