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.

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notpushkin 5 hours ago | parent [-]

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

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asddubs 4 hours ago | parent [-]

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

	▲	notpushkin 4 hours ago \| parent [-]
		Yes, but the list is for public suffixes, i.e. domains under which users can get their own subdomains.