Url in Extension and StructureDefinition


We are developing the FHIR inside our product.

Based on our business logic, we have the need to add extension to some Resources.
for the url under Extension, it states it SHALL be an absolute URL.

I understand the reason that we want this url be global unique, but situation is our product will be installed on various customer base, then it has different fqdn provided by customer.

So, we don’t have a fixed address which can represent the url.
then we need to give the url like

the url is likely global unique, but actually not accessible, our company don’t host a public fhir server.

is it acceptable from FHIR’s point of view?
If yes, wondering why we limit the url to an absolute URL, but an URN

Thanks in advance.

If you don’t want to host the extensions yourselves, the simplest thing to do is to put your extension in a public registry like simplifier.net and use the address assigned by that as your URL. Unless the extensions are distinct per customer, you wouldn’t want the base URL to be customer-specific as you’d like all of your customers to use the same URL for the same extension.