Kind of late reply, but for whoever reads this: Milk is not profitable, do not sell it on cloudy or overcast days (unless you have a bad reputation). Selling milk is always at a loss, but it prevents your reputation from dropping when you don't have a lot of variety in your products since at the beginning, if you don't have milk, that's a whole third of your customers that aren't satisfied. That's manageable on cloudy and overcast days because you don't get many customers, so that's fine, your reputation will not drop too much as you're still satisfying two thirds of your customers. However on sunny days, that'll just make you running out of stock faster, and therefore make your reputation drop way earlier during the day, which may lead to you not having many customers on the following cloudy days as your reputation dropped too low. 

What you want to do, is sell as little milk as possible, just enough to keep getting customers, but not enough for you to make too little profit on your sales, so it'll take way longer before the next upgrade (I recommend to get chocolates bars as the first update, as they are profitable and will make you loose less reputation to unsatisfied customers). Getting more shelves is not important on cloudy and overcast days, it is however crucial for sunny days since that means you'll run out of products later in the day, and thus will lose a lot less reputation.

Well that got a bit longer than I expected, but that's my strategy when it comes to milk in the early game :)