You're not wrong to send what you know your kid needs.
But when my ex and I split up, he assumed I'd send along everything he needed for our 2-year-old twins, but I figured: He decided life would be better separate from me - and that meant not only getting away from the things he didn't like about me, or the things he felt I didn't do well, but also forgoing all the things he enjoyed about me and the things I did take care of, for him. Many of those things I was pretty sure he never noticed or thought about, so I had a little pleasure at the idea of him discovering how much work (and shopping!) was involved in taking care of two special-needs toddlers... that, in fact, it ISN'T "just sitting around on your a** all day, not working"!
But I digress. Although my ex felt I was just being spiteful and my mother thought I was absolutely monstruous and neglecting my children, it seemed perfectly reasonable to me. He was also their parent. He wanted to parent separately. Why was I neglectful, if I expected him to go buy diapers and changes of clothes, the same way I did?
And now, 13 years later, I think it was essential! It helped us clearly define that our lives had truly become separate. I did not feel resentful that he was using me for those wifely services that remained convenient (the details of childcare), while he lived in his own, private, peaceful bachelor pad and dated whomever he wanted. And he learned to parent his children independently, as a single father should be competent to do. We became comfortable and confident in our separate lives and the fact that we're each responsible for our kids during our time - and now that that is defined, we can relax the rules and help each other out with things, when needed, without getting hung up on who owes what to whom, or whether someone's not pulling their weight.