I've created simple and free "Price match" module for PrestaShop v1.6 (but it works on PrestaShop ver 1.5 too). Extension places a price match request button in product detail page and lets customers to send you a request about lower price on the same product in other stores. In administration section you have possibility to process price match requests. You can disable/enable sending letters for customer and/or store administrator about getting new request...