Fluctuating Courier Costs

We wholesale our kitabs to bookshops around the country and courier them using _______ Couriers. The bookshop sends us their order and we then send the kitabs with an invoice which has the price of the books and the courier cost. The bookshop pays us after they receive the kitabs and the invoice. We do not tell them the courier cost at the time of them placing the order. The courier cost is only mentioned in the invoice sent to them with the kitabs. The courier cost is known to us at the time of dispatching the books. Depending on whether we package it as 1 or 2 boxes there is a price difference of around R50 – R121 depending on the area we are sending to. We do not charge an extra amount for packaging. We pass on the exact cost of the courier to the bookshop. Also, we normally tell the bookshop at the time of the order that we will be using _______ Couriers, but there have been times when we did not mention the name of the courier company before the order was dispatched. Is this ijaarah faasidah? If yes, how do we correct it for all the years we have been operating in this way?