For flowers, an alternative to the mason jar idea would be to plant wheat grass in terra cotta pots. Then you can add a couple of gerber daisies in water picks (those plastic tubes that hold just one stem; the pick version is pointy on the bottom so you can stab it into things.) The grass will hide the picks, making it look like the daisies are growing in the pots, but it will look more 'done' than either floating daisies or potted plants. Also, Wheat grass seeds are so cheap they're practically free if you get them in bulk from a health food store, and they're insanely easy to grow. you could tie a red gingham ribbon on the pot (less overwhelming than gingham tablecloths), and if you wanted to get kitchy, you could glue a couple of plastic ants to the pot too.
I actually did this for my wedding (I think I originally saw it in Martha Stewart Weddings), with terra cotta pots, wheatgrass and gerbera daisies. I had to fashion wire holders so the daisies would stand upright, I guess I didnt know about the little holder things at the time. However, I didnt grow my own grass. Instead, I found a local wheatgrass supplier and bought a bunch of flats for cheap. I'm sure you could find a similar person (especially since you're in california).
And I wouldnt worry about buying your stuff from costco. I bought my flowers from Whole Foods and they turned out fine. I did this after feeling cheated by florists and learning that the SF flower market requires a business license for wholesale rates.
Hope your plans are going along great, good luck.