On your PPC landing pages never show a popup that promises a saving on the second purchase:
Shoppers clicking paid search ads (excluding branded ads) have multiple tabs open and are reviewing multiple offers at the same time. They’re in “speed dating” mode. An offer that applies to their next purchase is a total waste. In fact, I’d argue, it hurts conversions because you’re reminding the customer that there is nothing special for them for this first purchase. And there might very well be a great deal available, or maybe your product is way better than the other tabs but the popup appeared the moment I landed and now I’m turned off.