Why would this be my boyfriends reaction to tipping a delivery driver?

So, my boyfriend left this morning to go help his brother. Took much, much longer than expected so we weren't able to grocery shop as planned (only have one vehicle currently). Ultimately causing him to have to order food and have it delivered so me and the kids could eat. He usually does NOT tip through debit card because we are so used to tipping in cash.

So when the driver arrived I handed her a $20 for her tip and she started bawling her eyes out. Complete hysterics and super thankful over $20, saying that her job doesn't pay for wear and tear on her vehicle or pay for gas. Anyways, just a super thankful woman. With that said, it was an awkward but fulfilling moment for myself. I felt like I had done a good deed.

So I texted my boyfriend and said "So I just tipped the driver and she was so above and beyond thankful that she actually cried." His response was to call me freaking out because apparently he already tipped her $8. So I said "Well, now she has $28 to put in her gas tank". But he was still extremely butt hurt that I tipped this woman.

For clarification - I currently work full time in health care. I work overtime most weeks. I bring in good money. His company shut its doors due to Covid and has been out of work for 5 months. I have easily $12k in my savings so $20 is literally nothing to me and ultimately, where I have say over my money, he shouldn't be pissed.
Why would this be my boyfriends reaction to tipping a delivery driver?
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