Johnny5needsinput I'd suggest you do exactly the same as you would do now.
If it was acceptable to give a note in a card that wouldn't mean it was compulsory. Not everyone would. Just as when people ask for no gifts people still give them.
So you would still do whatever you do now. Maybe no gift (I'm totally cool with that -the obsession with presents is ridiculous IMO and even worse that it makes people on low budgets feel bad), maybe regift something, make something - just do what you do and please please please don't ever feel bad about not giving the "norm" of some plastic tat.
Home made cards are by far the best BTW, if you have the time to sort it out. I try to do them but don't always succeed. I encourage my DC to make cards too, and I keep a stash of cheap cards from Tiger at home for when we can't (.50p each and they are really lovely).
Paying £3 a card each time is utter nonsense. And some people like to spend loads on wrapping paper/bags etc - I recycle what we get in, and use old artwork of the DC as wrapping paper if I can. I have a birthday of someone close/important nearly every day of the first 2 months of October. Even as a FT working single parent I couldn't afford to cover the cards if I was to buy one for each person. Thank goodness for home-made cards, Tiger cheapies, and FB posts for those living abroad.
The worst that can happen is a parent wouldn't send their child to a party because they don't have card/present - and I've read about both sides of the story here on MN. Let's never lose site of the fact that children's parties are for children to celebrate with their friends, and feel special. Presents/cards should be an afterthought - the icing on the cake so to speak, not the focus of the day.
Just to add I'm a SP too - the difference is I work FT. We aren't poor, we have enough but we are far from rich. My main stress point is time - I don't have time to shop for kids birthday gifts (in fact I rarely spend time at any shops at all), and if I did have more time I would spend it with my DC, not on shopping for presents for DC's classmates. So a note in a card is my idea of a fantastic solution.