I get that there is a history and that no one likes the ex, but the exchange here just isn't that odd and certainly not stunning.
Take it from here side for a minute. She doesn't know anything about the lunch, shopping or him being strapped for cash. She get a text saying can we postpone our gift exchange. She thinks, it's a gift card, takes 5 minutes to buy, no thought required, and asks why. Seems like a reasonable response. He replies I need more time to shop. It's pretty obvious that there is another reason, since it doesn't take weeks to buy a gift card. She thinks, he's just blowing me off at the last minute, after I went out and bought him a gift for our upcoming gift exchange. She replies I'll think about it. It's kind of rude, but so is abruptly changing plans to exchange Christmas gifts until well after Christmas without giving a reason.
If he wrote back, I'm going out of town, strapped for cash, busy at work, maybe it would have not gotten the response of I'll think about it, we just don't know.
If an ex who I was friendly with suddenly started to blow me off for no reason, I'd rethink what was happening. Since it's not just a meeting to get the gift, but a gift exchange, for all we know, the I'll think about it could mean I'm now debating whether we should exchange gifts at all and I'm going to return the one I bought for you and spend the money on myself.