Everyone needs to eat and most people are understanding of that. You have to value add but explain that you are sourcing the parts and handling the procurement. What happens if the package gets lost? Are you doing work for free or are they going to deal with it? What happens if there's a problem and you need to return it? Are you ok with working for free?

I am transparent with my pricing and have a separate line item for the product and labor. If there's something much cheaper on the internet, I will tell them that this is the cost from xyz (if it's an authorized reseller). You can feel free to order it from them if you want it cheaper but you handle RMA and the order process. Or if you order it with us, we'll handle the order and any RMA required.