Digital Koya Heritage Ticket and Ltd express

Hi, I have a specific question about the Digital Koya World Heritage Ticket (so many things are changing rapidly regarding train tickets), if I want to board a limited express train from Namba to Gokurakubashi:

given that a Limited express version of the pass is not available, is it possibile to buy the limited express ticket online and reserve a seat or do I have to phisically go to the ticket booth once at the station?

I see there is a section on Nankai's site that lets you reserve Limited express tickets, but that is meant for users that don't have the Koya World Heritage Ticket and I suppose it's more expensive.

Can someone please clarify this to me?

thank you

As you pointed out, there is a website that allows you to make seat reservations, regardless of whether you pay the base fare with a digital pass, IC card or paper ticket:
https://www.club-nankai.jp/traffic/ticketless/en/

It is not more expensive.