One particularly pleasant service is NanaGram, which will mail 10 4x4 or 4x6 photos of your choice to your grandparents — or any relative, really, but it's cutest to say grandparents — for a fixed rate of $7.99 every month. (This is the most basic package, but you can also pay more to send more photos.)
NanaGram does not have an app — a nice perk for those whose phone storage is perpetually full. Instead, users text photos with captions to a phone number connected to their account. This also means that multiple people (siblings, cousins, etc.) can use the number, distinguishing NanaGram from similar services. It's like sharing a Netflix account, but with your relatives and for a kind reason. Nice!