Charcuterie for 1?

At Monprix you can find some little pre-made packs. Don't be afrad to try a nice terrine de campagne as well.
At a chacturerie they might be slightly irked if you're getting so little, but you never know. Explain you're going on a picnic and just want an array of choices for one.

La Grande Épicerie de Paris (38 rue de Sèvres across from the Bon Marche department store) often has small trays of mixed cheese wedges, and small trays of mixed charcuterie slices, suitable for 1-2. They are catering to the young single professional crowd shopping after work. It’s a small enough serving that it should be ok for one as lunch.

If you have a minibar in your hotel room, of course, everything would be easier.

Please, do me a favor, don't go buy pre-packaged charcuterie in Monoprix or other supermarkets!!
Unless of course if you like the taste of the different chemical additives that are there :)

You can go to any traditional Charcuterie or "Boucherie-Charcuterie" (you will find some in the 5th)
You can ask for just one slice of ham (Une Tranche) or two or three or whatever you want, you will be asked for the thickness. (Epaisseur).

The same for terrines or other pâtés or sausages and pretty much everything you'll see in the shop.

Thanks everyone for your ideas!
Yes, I’m very familiar with Bon Marche.
Love the suggestion to make sure I don’t make request during a busy time for staff! Definitely would prefer fresh vs. prepackaged so will try that.

I'd say, don't overthink it... A quiet charcuterie won't scold you for asking 2 slices of ham and a few other things. And even if they do, they'll still serve you, even if begrudgingly... I would certainly not make the trek to Bon Marché if it's not close to you.
By the way, Monoprix stores have both prepackaged stuff and staffed counters. Staffed counters are usually pretty quiet, and they do sell good stuff, even though some of it will be mass-produced.