Abstract: A purchaser user enrolled with a service provider for promotional sales is provided with a smart cell phone and possibly a bank card issued by a banking partner of the service provider, said bank card being associated with a debit/credit account for a means of payment for promotional sales of the service provider. After having been grouped together the purchasers make their purchases within the framework of a grouped promotional sale using purchase coupons issued by the service provider to the cell phone of each purchaser user. Said user pays for his or her purchase while benefiting from a first discount. The service provider reconciles the cash register receipts and the purchase coupons, calculates a second reduction as a function of the validated purchases of the members of each purchase group and credits, if appropriate, the members of each group with said second reduction.