How much difference are we talking here?
Remember that your X will then take the child to therapist about mental anguish over mom suggesting that child should have gotten horrid glasses.
You X can get in the same leaky boat as I've wanted my X to be in.
The billing/payment issue should NOT involve the child .
Dad made a choice ( perhaps the child liked the glasses - this could also add to the child's self esteem ) .
Your call if you want to contribute .
I agree though the covered frames these days aren't terrible .