To lend money to customers on 5% or to pay a company for the customers and take 5% additional money from the customers, both the cases are of interest which is unlawful. Yes, you may deal with customers thus that they should inform you after having checked the things in any shop and then you tell them that I will sell you those things after purchasing them on 5% increase. If he gets ready to buy this way, you are allowed to take the profit. There will be a condition that first you possess those things by buying and then sell them to your customers.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband