Hi, custom postage is claimed from Royal Mail, not eBay, so you did the right thing. I'm a little confused as to whether you used 48 tracked or large letter, I assume tracked as you have tracking info. You may as well use SD if you use 48 tracked, you can deselect evri. 

Either way, they tend to only send stamps as compo for low value items and it can take literally months to get a reply. Last one I did was not that long though. I have had a couple where they said they sent stamps, but I never got them...... Did they lose those as well?