I agree pre-stretch is probably pretty standard. String builders have to stretch them to get the finished product to the correct length. If a certain maker doesn't pre-stretch then I would pay the extra money.
And put the new strings/cables on yourself. It's super easy. I use a cheap bowmaster cable press to relax the bow. The press is a must have for a ton of other things too (peep installs, cam timing & lean adjusting, draw length mods, etc.). So just get one. It will save a ton of money and time in the long run. They only cost about $30 - $50, and well worth it. If installing your own strings just make sure you don't let out any twists before or during the install. The mfg will probably clip the string ends together before shipping so they won't untwist. Keep them that way, otherwise the string lengths won't be to spec. Once they are on your bow then you can tweak them for timing/tuning purposes, but at least you have the base mark to start from.