If they're typical wedding tracks, just put them in a playlist and mix them live? Then if they change their mind on the order, no time wasted. If they're requesting common songs that get played at weddings all the time, you should probably already have them, and that's on you. If they're requesting obscure stuff that will take time to download and add to your collection and you'll never use it again, make that clear to them and say you can spend up to x time (whatever time you're prepared to spend for free catering to their needs), then after that charge for your time. Nothing comes free.
As for recording premixes, that isn't in the job description, and should be made clear. Just mix it together on the night.