Map the background and paint it first. Once the background is dry, paint the bottles, a trick is to draw the elements, using a dark color, over this drawing you paint the background in a semi transparent fashion, this will be your map to continue painting the bottles last.

I would first draw an outline, then start painting. You can paint in the background as you go. You’re right. Oil paints do often mix, and people use this if they want to blend something. If you don’t want any mixing, you can glaze over your bottom layer. The undercoat must be completely dry before you add the next layer, and the paints won’t mix at all. Oil paint takes a long time to dry, so you can add Liquin or galkyd to your paint to make it dry quicker. One of the top layers should be a thin transparent grayish white to give it those hazy milky tones. Also, you can add a nice burgundy transparent layer on top of the dark bottles for the reflections of wine in there. Also you can use a medium like Liquin to make your paint thinner and transparent and quick drying so you can paint your next layer over it. I think you can base the price on how good of a job you think you did after it’s finished. If you are a beginner, but you are really good, and your customer is happy with it, charge $99. If you have purchased a standard white canvas from an art store, you don’t need to put anything over it. Only if you make your own canvases, and have raw cloth would you need to paint something to cover it.