Is a transaction considered usurious if it involves sending customers to contracted stores to purchase goods using the company's money, then the customer repays the company in installments without any increase over the cash price of the item or a late payment penalty, in exchange for the company receiving a percentage of the item's price from the store?

It is permissible to take a commission for guiding customers to shops, as it is a form of brokerage (samsarah) and an incentive (ju'alah), both of which are not prohibited.

However, your act of paying the price of the commodity on behalf of the buyer, then collecting it from him in installments without profit, while also taking a commission from the merchant for guiding the customer, is not permissible. This is because it is a loan that draws a benefit (qard jarr nafa'an).

The way out of this is for you to buy the commodity specified by the customer, then sell it to him after it has entered your possession and responsibility. It is permissible to sell it with a profit and to divide the payment into installments according to the agreement.