Contract Law: Brokerage / Commission Invoicing

My question involves business law in the state of: Florida, Nationwide.

I am trying to determine how to legally invoice a customer for a product and "service fee" preferably with one line item.

For Example;

A customer calls me and says, "Joe I need a sack of potatoes delivered to my pig farm in Colorado".

In turn I call the potato stand in Colorado nearest to the customers pig farm and order one sack of potatoes delivered to the customers address. I pay the potato stand $25 INCLUDING all sales tax, USDA Inspection taxes etc etc...

Am I now allowed to bill the customer $30 for "Service Provided"? (no sales tax since 1. It's already been paid, 2. I billed the customer for the service, not the product).

In more detail, the customer and I signed a contract prior to delivery stating the potatoes are his responsibility and I am held harmless from all issues that may arise from the potatoes.

Can I purchase something "IN CARE OF CUSTOMER", then bill the customer for my reimbursement plus my fee all on one line item? Or at least in a way that the customer would not see that I made $5? I would need it to appear that I was not reselling the potatoes.

Any help, input, creative insight would be greatly appreciated.

Thanks a lot!
Joe

Contract Law: Brokerage / Commission Invoicing