[c0balt]

You can, but is difficult and prone to errors. Separate domains solve the root cause of the issue. The alternative is an entry on the public suffix list.

[notpushkin]

Which would not be easy to get, considering PayPal is not running a public suffix.

[asddubs]

you can request entries on it, the list is not just for TLDs

[notpushkin]

Yes, but the list is for public suffixes, i.e. domains under which users can get their own subdomains.