If you do not want to show the details of material costs on an invoice, then there is a way to easily group the material lines on a Job into a single line item on an invoice.
When creating a Time and Materials based invoice, simply place a tick on the check box option, "Group Materials into a single line item on an invoice"
This will summarise all the material lines on the Materials tab on the Job into a single Misc Line item on an invoice. (See example provided below)
Job Material line details, both Price List Items and Miscellaneous Costs:
Combined into a single Misc Material line item on the invoice:
The invoice line description can be altered to read as required.