You can top up the £ 15 "Clever Cup" using Barclay's bPay app and then use it to pay for your (large) fill of coffee, tea, caramelized orange hot chocolate, or whatever else takes your fancy. And its powers are not limited to Costa's own stores: you can tap to pay with the cup wherever contactless payments are accepted. (Just try not to destroy a hapless grocery store's chip and pin machine by spilling your flat white all over it).
Of course, contactless coffee purchases were heralded in by the mega-popular app of Starbucks, which was recently crowned as the biggest mobile payment app in the US, beating out the likes of Apple Pay and Google Pay. Costa has an app too, in case you forgot, though it's just for amassing Coffee Club points.