I'm no longer with Booktrope (personal reasons...nothing really to do with them), but technically you DO have to pay.
From the "Author FAQ" sent to me after my novel was accepted:
We do not charge up-front fees, meaning, you are not asked to pay us to produce your book. We also do
not have a minimum (or maximum) amount of print books which you are required to purchase. We do
“charge back”, meaning deduct before paying royalties, the set-up fees to the entire team, with the
rates shown below. This is in order to ensure we are able continue to publish books as cost-effectively as
possible. Please note that Booktrope does not recoup our entire expense, and we certainly don’t profit
from this!
Set-up fees at present are (these are subject to change):
Ebook - $50
Paperback - $99.50
Paperback and ebook - $149.50
Republishing of existing title - $80 (i.e. content update at author request)
And with our new Frequent Flyer program for repeat authors set up is $75 for both titles (details
in Teamtrope).
I don't know if that's standard practice or not, but you ARE being charged.