I know this is an old thread, but it's posting about a Christmas bizarre.
Adding a tin to package the magnet in when it's sold is going to add up to your costs considerably.
There are plenty of suppliers that sell them. I shop at SKS for some of my containers and they do have good prices compared to other vendors. But metal jars are not cheap..... http://www.sks-bottle.com/340c/SearchIndexMetal.html
But why not just keep a roll of bubble wrap close at hand, you could pre-rip them into sheets and a roll of tape to quickly wrap the magnet up and secure it with a bit of scotch tape? It would protect the magnet until it's arrived home, and be a bit more cost effective.
Buyers might not like the extra cost of the tin, and then not really have a use for it after. I guess to me it would seem odd packaging them in a tin after purchase.....
You could also print out a label on a sheet of regular size address labels and quickly pop on onto the outside of the bubble wrap. That way it wouldn't get lost unlike a business card could.... to advertise your items if you sell them online as well. You never know they may be a repeat buyer or buy as gifts.
I use a purchased program for most of my labels but for labels for round jars that are the same I do use this program. http://www.onlinelabels.com/general/maestro-label-design-software-pricing.aspx
If you have your own labels and print for free, it does print words on the first row of labels, so you do have a bit of waste that way. But I also buy my labels from them so I get the program for free with my purchase of labels. You can save you designed labels and then print out an entire sheet of them. And you can do some pretty neat stuff with the program! Just some food for thought.