it wouldn't really make sense for the user to specify false, tho they could. In this case it's because bool is the most effective type for the purpose of this param, but we want it to default to false. So in practice, the user --- in this case, a react dev on Cashtab -- either specifies showToast or does not. If it is unspecified, the component defaults to false below.

It's a common react convention. In this way, you can specify <CopyIconButton showToast/> and you will get this as true, no need for showToast={true}

the only user-input field is quantity, which is validated (must be a number, only 1 decimal place, decimal place must be '.', etc)

The string itself is built programmatically in the component. So, right now, the only possible options are bip21 firma token + qty and bip21 xec address (optional qty).

It probably is possible to get isBip21 as true with bad input in the field. But in this case, it just means the user display would be bip21 query string copied to clipboard instead of the much longer actual string. imo it's ok to copy paste "bad" strings like this ... I guess we could disable it on validation error. But Cashtab and electrum already validate bip21 strings, so would see it there.