[kmfrk]

I think the best way to go about it is on a per-occasion and -person basis. A Mother's Day bouquet and card is different from a wedding RSVP.

The best way to frame this product could perhaps be in terms of convenience and design rather than personalization. This scales really well for "batch" jobs, if you're sending thank-you letters or answering fan mail, for instance. In that case, there isn't really a problem with the quasi-analogue aspect of it, especially since people know that it's taken the person time and labour to craft the letter.

The tricky aspect of it is that you are basically selling a product to two different demographics: the sender and the recipient. And getting the first one on board is, as you say, not that hard with such an interesting product.

A good rule of thumb could be to use this in general, but be cautious of using it for annual events where it already feels like we are forced to extend cultural tokens of gestures towards each other?