two things

[artman325]

it's easy to add these conditions

1. don't exceed maxTotalSupply
2. make a method buy() which will transferFrom(msg.sender, token, amount) and mint-send ITR or revert if exceeding maxTotalSupply

function buy(amount) {

if (buyPrice == 0) {
   revert BuyNotAvailable();
}
if (buyToken == address(0)) {
   _mint(msg.sender, msg.value / buyPrice); // ignore amount
} else {
   IERC20(buyToken).transferFrom(msg.sender, amount);
   _mint(msg.sender, amount / buyPrice);
}

};

function _mint() {
   if (maxTotalSupply > 0 && totalSupply > maxTotalSupply) {
        revert MaxTotalSupplyExceeded();
   }
}

[artman325]

The changes were implemented in the "43-two-things" branch, but the code exceeded 24 KB. The task will be considered finished when #44 is completed. There were a couple of things that will be unnecessary and will be removed.