If you are effectively just ordering something, I'd say: Keep it short and sweet. If I wanted to have Coldrex, and see it right behind the counter at a drug store, I'd simply say: "Een (klein/groot) pakje Coldrex [type], alstublieft." It may be direct, but it works. "alstublieft" shows plenty of respect, in my mind. This is the same thing what I say when ordering something to eat or drink at a train station or so, picking something of the menu. Ron is right that you could say "Mag ik ..." or "Ik wil graag ..." but I think most people would consider it overly verbose. Especially "Mag ik ..." may raise eyebrows. I think most people will be like "of course you may!". Effectively it forces people into a quite purposeless conversation, however tiny. At least that's how I would think of it. Actually I don't really know how it would be taken by others. I'm gonna test this out the next few times I order something. It'll be fun! "Ik wil graag..." would be more appropriate if you don't know exactly what you want yet. "Ik wil graag een pakje Coldrex kopen. Wat voor soorten heeft u (allemaal) (precies)?". I think by itself, "Ik wil graag een ..." sounds less polite than "een ... , alstublieft.". While the Dutch are said to be blunt, I think most people much appreciate politeness. "alstublieft" en "dank u wel" just happen to work very well. Don't leave them out.

You can certainly say: "Mag ik (van u)...", "Ik wil graag ...", "Ik ben op zoek naar ...(if you cannot find it)","Heeft u ook..."(if you are not certain if the store has the product). Dutchmen are very direct and according to foreigners rather rude, so don't be shy or afraid you are too direct.

There are different possibilities to formulate such a reply: 1. Ja, ik zoek het medicament "Coldrex". 2. Ja, ik ben op zoek naar het medicament "Coldrex". 3. Hebt u het medicament "Coldrex" op vooraad?