Our secondary sends out postcards for many things, including to children who aren't even pupils there yet as they send messages welcoming new pupils before they start and saying they are looking forward to meeting/teaching them.
Our boys have had various postcards for a variety of things, some printed and some hand written. Some are to invite them to extra curricular events (hopefully connected with school things and not personal!), some to congratulate on achievements, some thank yous, some other things.
Ds2 had a personally written postcard from a teacher once thanking him for being such a good pupil and for helping her in some lessons. When we saw her later at a school event she even saw us personally and mentioned it to us as he'd been such an encouragement to her. Turned out one of her classes ds was in were not the best class at the time and ds had made things easier by behaving, helping and just doing his work. Encouraged her through the rough patch until she got a handle on the class. She hand wrote a note as it meant so much to her.
Last week another teacher told us ds1 had made her a lovely cake topper for her dds birthday and she hadn't forgotten she'd promised him a bar of chocolate as a thank you. Turned out ds had used the laser cutter to make something for her dd (when calibrating/testing the machine he'd cut out something for her rather than a sample cut that goes to waste - he's sixth form and gets to help maintain these things) and she bought him some chocolate. He came in the beginning of this week with a big box of chocolates, not a £1 bar of Galaxy! We didn't think 'That's a bit much, she's overstepping her boundaries' but did think 'Wow, that's generous/kind of her, where's my chocolate ds?'
Unless something is abusive I think it is best to look at the good side of it, rather than look for a downside.