In an introductory note to this chapter, the Kitzur Shulchan Aruch writes that most Jews are already familiar with the prohibited labors of Shabbos and that he is only addressing common situations with which people may not be familiar. Clearly, the laws of Shabbos constitute an area requiring much greater study than this email alone will allow.
80:65 On Shabbos, one may instruct a vendor he knows to fill a container for him. This is true even if it’s the kind of container specifically used for measuring and even if the container belongs to the seller, which the buyer will borrow in order to take the produce home. All the more so it is permitted if the buyer brings his own container. It is not permitted, however, for the seller to measure in a container specially used for this purpose and then to pour the contents into the buyer’s container. (Rema 323:1 permits this so long as the seller does not intend to measure out a specific amount.) Similarly, one may ask a vendor he knows for fifty nuts or some such, so long as he doesn’t mention any particular measure and they don’t discuss payment. One may also not calculate the total, e.g. by saying, “You already gave me fifty nuts; give me another fifty and I’ll owe you for one hundred.” One may certainly not use sales terminology in these dealings, even if the parties don’t mention the price and even if the items in question are needed of Shabbos. One may not have a non-Jew make purchases or rentals on his behalf on Shabbos.
80:66 One may tell a vendor he knows, “Fill this container for me…” or “Give me up to this line and tomorrow we will measure or weigh it.” (One may not normally plan on Shabbos for after-Shabbos activities but it is permitted here because the item in question is needed for Shabbos – Mishnah Brurah 323:8.)