I've seen that there is a purchase button to buy with scrap, but can this be directly linked to my website so that a player can purchase the kit with real currency rather than in game currency?

do with the command from the web store like this.

kit give {userid} KITNAME

If your kit name contains a space, e.g., VIP Package 1,

You must write it like this: "VIP Package 1."

Example :

kit give {steam_id} "VIP Package 1"

user-id depends on what store you use.

On Tebex, I believe it is {steam_id}.

You can read their documentation to point to buyers' SteamID.

Your kit must have permission if you want to show it on the kit menu, but unclaimable, let's say the kit permission is kits.buyable, but don't give anyone this permission. No one but the admin can claim that (can be edited in the config for the admin to bypass any permission for kits).

if you only want to show it on your web store. Simply make it hidden.