S20 rear cover and battery adhesive

When you say "I search your site", that gives me the impression you're trying to talk to the people who handle the iFixit Store and their inventory. Unfortunately, you're on the repair forum, manned by volunteers and users, not iFixit employees, and to the best of my knowledge, the Store personnel don't hang out here keeping an eye out for Store-related questions.

If you really feel the need to contact them, you'd best go through the FixBot assistant or email them directly at support@ifixit.com.

That being said, from what I can tell, the SM-G780 and SM-G781 are the same phone with two differences; the G780 has an Exynos processor and only supports 4G wireless, while the G781 has a Snapdragon processor and supports 5G. There are minor differences internally with the addition of an extra antenna, but the display and back glass appear to be the same.

The letters at the end of the part number carry region-specific information and also indicate if it supports dual SIM cards, but otherwise have no effect on the parts you're interested in.

At least one seller on Amazon carries the back glass and advertises it as being for both the SM-G780 and SM-G781.

Amazon.com: Galaxy S20 FE Back Glass Cover Replacement with Pre-Installed Camera Lens + All The Adhesive + Tool Kit for Samsung Galaxy S20 FE 5G SM-G780 SM-G781 All Carriers (Cloud Lavender) : Cell Phones & Accessories

Personally, I wasn't able to locate a back glass on the iFixit site, but I'm only searching the US site so other regions could be different.

That's the best information I was able to come up with, but again for an authoritative answer you'll need to contact the iFixit folks directly; I don't work or speak for them as I'm just a volunteer moderator.