Unlike in the old west, the slow draw wins…
But seriously, it’s better to say something at the moment rather than to remain quiet and grumble about it later.
Maybe your friends are/were on a tight budget and were embarrassed to say so. I’m thinking some kind of drastic reduction in their income brought on by the economic crisis; possible some bad news received after the plans had been made. The nature of your plans (supermarket, park) suggests that some or all of them were on a tight budget.
I think some posters are misinterpreting the OP’s comment regarding the advice given by his/her parents. If I’m not mistaken, what he/she is saying that if he/she had been in the position of his/her friends, he/she wouldn’t have hesitated to offer payment each time.
In any case, now you know better, right?