It can be confusing when reading these veggies articles, since some are contradictory. The link that I gave before, from the House Rabbit Society, seems to be saying that for every 3 greens that you give daily, only one should be from the first list (high oxalic acid content). So, if you gave a variety of 6 greens a day, 2 could be from the first list and 4 could be from the second list. At least that’s how I interpret it. Instead of going so much by numbers, I think it would be easier to have several kinds of greens on hand from the second list, but just one or two kinds of greens on hand from the first list, and when you make your salads, go heavy on the greens from the second list and go light on the ones from the first list. Your rabbit will have preferences, and you can determine those as time goes by and you try different greens.
I do buy prepacked spring mix, usually the organic brands. I first look for the best dates, and I won’t get a package that’s less than 5 days away from the use by date. It try to get ones that are 7 or more days from the use by date. Even with the very best dates on bags or containers, there always seem to be some bad/slimy pieces. And some never stay fresh through that date. Some brands seem to be generally better than others. So you have to carefully look at the pieces and not just grab a handful to put out for your rabbit. Some bags or containers I can tell aren’t going to be good right from the start because the lettuces are too wet. I find the best thing to do is a day or two after opening the bag or container, I put down a paper towel and sort through the bag, placing the good greens on the paper towel, then back in the bag, and the bad ones get put in the compost. The bad or too wet greens will start sticking to the good ones and making them go bad.