Forcing you to spend more

I wished to purchase an item at the store. Upon paying for the item I request to pay for the item with my debit card as well as pay an instalment on my account. These instructions were ignored and the item was put on my account. I then asked again to kindly assist my charging me separate for this item and then combining the instalment amount as on amount on my debit card. 2 different floor managers later and highly irritated cashier later it was told to me that this was impossible and that cash sales had to be charged as a separate charge. Surely its logical that the same amount ends in the till. It just seems to me that a bit of laziness has crept into the book keeping of Mr. Price to the expense of the customer that has two pay for two bank charges now or extra interest if the amount is put onto the account. I left both paying my instalment and the item I wished to purchase. I would appreciate a clearer explanation behind this arrangement which leaves the customer out of pocket.