Yea, maybe not a shirt, but hell, send them some nice coffee or some snacks or something. My current job is probably at least 50% implementing stuff my company bought, and for one of them, they had an on-site meeting about the implementation/proof of value stuff and they offered to buy everyone coffee/tea/whatever at a coffee shop by our office before the meeting and that definitely helped me be a bit less grumpy about the work.
Any sort of "congrats on launching/implementing our stuff" gift is at least an acknowledgment of the work put in to help the sales team land their contract, and that helps keep the relationship on a good footing.
A few companies I have worked for have had a strict no gifts policy. Many of our customers had similar policies, to the point that we couldn't even pay for a customer's coffees if we have a meeting at a cafe.
These policies are aimed at preventing even the whiff of bribes or favoritism wrt purchasing or awarding of contracts.
Mostly this was in the Oil and Gas industry. We had a few mining clients that were less strict wrt gifts/swag. I now work in the banking industry where there are strict regulations against bribery, facilitation payments and many types of gifts.
So I find it a bit weird to hear that sellers provide "swag".
Any sort of "congrats on launching/implementing our stuff" gift is at least an acknowledgment of the work put in to help the sales team land their contract, and that helps keep the relationship on a good footing.