Redirect to URL Action with Aggregate Variable Doesn't Work?

I have a page with a model that holds a user's list of courses they're registering for, each with a price. I then have a model that uses the same data, except in the aggregate, in order to get the sum of all courses. I'm then trying to redirect to a PayPal URL and supply that total to the URL string.

I have tried both:

http://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&business=XXXXX&lc=US&item_name=Test&amount={{{sumCourseOfferingrMemberP}}}

and:

http://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&business=XXXXX&lc=US&item_name=Test&amount={{{$Model.RequestTotals.data.0.sumCourseOfferingrMemberP}}}

If I use:

http://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&business=XXXXX&lc=US&item_name=Test&amount=123

It works fine. If I include the same variable in the button label, it works fine. Are Redirect to URL actions unable to read aggregate fields?

My next step is to write a snippet that copies that value to a real field and read from there, but I'm hoping to not have to go that route.