With this, I could only say that you don't even know what you replied to.
I perfectly understood your reply.
Custodial wallets, whether an exchange or not often let you send coins for free within their network, which was what I pointed to in my reply. Maybe you did not read it to understand. And I added that such must be popular to increase the chance of OP's customers using the same to enable free transactions for some of them. I've used this on some custodial wallets and Binance (exchange) to third parties for free, so I know what I wrote.
If talking about businesses, custodial exchanges should be left not talk about. All I can see about this is how the people should depend more on centralized exchanges.
As for your suggestion on the payment processors, they are the most convenient, but maybe you don't use them, I do weekly, and I will always discourage a small/startup business to use them because they will not grow due to high charges. They extort customers because they set the price and fee which is beyond the control of the seller. This discourages buying and selling through cryptos as experience people will prefer fiat in such cases (if there is an option for it) to crypto in which they will pay more.
It is not about conveniences, when talking about payment and customers, it is about considering all means of payment, while Marchants are using noncustodial means to accept payment.
Payment processor is not for people that are receiving small number of transactions, you can handle that yourself on your noncustodial wallet, but it is used for high number of daily transactions with different addresses generation and also able to process your coin to fiat transaction for you as easy. There are many organizations today that are making use of payment processes if they are receiving high transaction rate.
As for exchanges, a coin can be tainted, the fault may not even be from your customers, while exchange may seize the coin. If taking about customers paying you, exchanges and anything custodial should not be talked about at all. Anything centralized may bring inconveniences, even up to withdrawal inconveniences.
Exchanges should not be more than exchanging money. For payment, payment processors can handle that.