Point one. They are not divorced parents. They are still married to each other, and gifting to their ‘child.’ Presumably they do the same for the others, too.
Point two. It is less hassle to send one amount of money abroad, and cheaper. Perhaps knowing how mean he is, she chips in his half, as well as paying her own £15.
Point three. Men are notoriously good at letting their wife deal with gift-giving and card-sending. I exbpect that he also lets you do it for any other family occasions.
The amount that they are giving is irrelevant. I’m far from mean, but my ‘children and I decided, years ago, just to send a card to the grown-ups. Trying to choose suitable gifts is difficult, and we often get it wrong, which is a waste of money, and embarrassing to giver and recipient. Postage costs are also horrendous, and a waste of money. Sending cash back and forth also seems silly. I give generously to my GC, and receive a small token gift, such as flowers, from them - which is all I want.
I think that the thing that upset you, as others have pointed out, is the fact that your opinion was over-ruled by his wife. I suspect that she didn’t even know that you had expressed one, and perhaps a ‘token’ £30 is their usual gift to their children. He is certainly guilty of being thoughtless and tactless in dealing with the matter, but he probably had no idea that it would upset you. He and his wife may have had this arrangement since before they separated, and have just continued to do it in the same way.
I would forget all about it, and concentrate on getting him to loosen his purse strings. His family may well inherit the lot, when he goes, so he shouldn’t begrudge you a few coffees.