No seriously a cheap waterproof rain slicker or pair of rubber boots will bribe the average person very easily, waterproof materials were nonexistent. It is also really easy to demonstrate and you can buy them at a dollar store. The best they had at the time were merely water resistant materials which tended to stain since they were soaked in oil. Plus there is no risk of accused of black magic like you might with medicine or any other active chemical. Plus it will rot away in a few decades so no evidence, as long as you don't pass out hundreds of them.
Nails were valuable enough to be major market item becasue they were time consuming to make. But at the same time, they were common enough that anyone would recognize and want them. Small furniture nails were even more valuable. A box of nails both valuable AND easy to fraction out (20 nails for a piece of whatever I smell cooking), and would not draw that much attention, since they did exist. you can buy square/box nails at any large hardware store, The process is different but the finished project will be identical.
Salt is just easy its desirable and easy to come by, sea salt is considered more desirable, spices are more tricky. You could easily be accused of selling poison or being a smuggler or get your friends accused of the same.
Cotton Go buy a few bolts of cotton cloth from Walmart. You can trade each one for a month of food and shelter without much problem. Go with tan or off-white to reduce suspicion. Remember: you don't want to give out too much wealth because it just makes those people a target. Cotton was known at the time, but it was traded from India at a high cost. Just buy "all natural" or unbleached cotton.
Wooden dice Easy to carry, dice were common, boredom is universal and you can buy them at any craft store cheap.
Sewing needles Light, easy to carry, you can buy them at Walmart by the hundreds for pocket change and everyone in the medieval society will see their value. Go with big over small however, the cloth of the time was rough spun.
Combs Even metal combs are cheap, pretty, and combs are universal so that anyone with hair will see their value. Just get simple designs and any archaeological evidence will be too corroded to look out of place.
Mink oil made of mink oil and lanolin is used to waterproof leather. Waterproofing would always have been valuable at the time and both ingredients were available and used. They were just a pain in the ass to get in any quantity. Heck, you could just buy a tub of lanolin at any fabric store - years worth of work in one tub!
As Zwol mentioned many goods had monopolies or defacto monopolies (guilds, charters, patents, ect) in certain countries so you could get your friends in serious trouble, so far as I know none of these had monopolies at the time although some did develop ones several centuries later. This is also why I left things like purple dye off the list they were far more restricted and not something you want your friends to be caught with.
For more ideas, I recommend the video series "worst jobs in history" to get an idea of what was valuable at the time.