I have just recently began doing custom covers for all of my blu-rays. I came across a site called 9 Covers (http://www.9covers.com
). It costs $20 for a year but there are lots of members who do covers and for me, $20 was well worth it. The site admin personally examines all the covers that are uploaded to make sure they are high quality and meet site specs. Every single cover I have downloaded has looked amazing and they have a great selection. If they don't have what you are looking for, cover requests seem to get filled very quickly as well.
I then realized I had no good way of printing these off however. The printer I have is not good enough to print high quality images and I didn't feel like going out and buying a $200-300 printer and the cost of ink. I searched online for sites like Office Max, Staples and Kinkos but the cost was around $1.00 a page for glossy paper. I then came across a site called http://www.moxicopy.com
. When making my order, the copies came out to $0.20 a piece on 80# glossy paper. I did a test batch through them of 5 covers and they came out looking great! I just recently made an order for the 110 covers I had collected for all of my blu-rays and with shipping it was only $22. Their customer service was also very good. On my most recent order, I had forgotten to include 6 covers in the original file I uploaded so I emailed them and was able to get them added with no problem. Lastly, I also requested for them to cut the covers for me and they did that at no additional charge and I couldn't even tell that they had been cut from a full 8.5x11 sheet.
The way I upload all of my covers is using UnderCoverXP which was mentioned in the OP and print to a PDF. I then use a PDF merger and merge all of my covers into one PDF file. The file I uploaded for my large order was over 80 MB but their site had no problem accepting it.
This is all just my own experience and the way I plan on doing custom covers for now on but I feel it is a very easy and cheap way of doing it. I recommend either of these two sites to anyone who is looking to do custom covers.