How to offset an expense which is paid back in clients invoice

Hi, 

I use QBSE. 

I work in a medical field where I often buy items say from Amazon or medical sites, for a client, rather than getting the client to purchase it themselves. This is because I work with people who have had brain injuries, so them ordering the item they need is too difficult.

I order the item, and add the cost onto their monthly invoice - I make no money off of the item.

This however is then paid as part of their monthly payment to me, from my invoice.

I need to then offset the cost against the invoice - what is the best way to do this? Do I invoice separately for this, and then exclude the income on Quickbooks, or is there a different way?

Thanks 

Amber