OK, sounds like you need a few calculation fields.
Go to File | Manage | Databases, find your table and create the fields you need, for example Grand_Total_80Disc would be specified as
YourTable::YourGrandTotalField * .8
this field then can sit on your layout instead of / as well as the Grand_Total field - then much the same for 20%.
The calculation for the 7% maybe slightly trickier - you'll need to supply more detail about how you select Org Services, is it an add-on after the GT calc or before?
The first calculations you sent worked!!!! Thank you so much!!!
As for the 7%, the Org Services is before, part of the Grand Total. It is a line item in the invoice. I have added a screen shot. Thank you!
In FileMaker 12, you can define an SQL Query to use with the ExecuteSQL function that returns the amount of the related line items where the product ID is the product ID for your Org Services. Multiply that result by your decimal equivalent of your percent like you do in the other cases and you have your calcualtion.
You can also use a new Tutorial: What are Table Occurrences? of the line items table and set up a relationship that matches by InvoiceID and by the ProductID for the Org Services product. You can define a calculation field ni Invoices that always returns the ProductID for Org Services to use as a match field in that relationship.