Apologies for the late reply!
There are many ways mail merging can be used, depending on the tables you're drawing upon.
Depending on which screen you are sending the template from, will determine which table to pull the merged fields from.
In your case the table is Estimate. So you'll want to pull a field from the Estimate table.
Try this:
Estimate.Customer
Also worth taking a peak at the following Help Center article as well:
Mail Merge
Thanks!
-- Morty