Retailers, like me, do it to build traffic and expose potential
customers to the goods on display. That probably wouldn't apply to an
internet seller. We also do it to build good will, that most
definitely applies to you.
Consider a thank you package...send them a shipping box/label that
they can use. Enclose any merchandising materials you think
appropriate. They pay all shipping and insurance. How neat is that,
they pay the cost of you promoting yourself! The cleaning service
itself is free. You may sell repair service because often you will
find loose stones etc. If you don't want to handle the repairs
yourself you can farm it out, hopefully there is a local trade shop.
And don't be afraid to charge for it, triple key on smaller repairs
is normal. This can become a real bread and butter income source, and
every business needs the bread and butter. This would probably not
work well with people who haven't bought from you, but those who are
asking seem to already have faith in you, Take advantage of that
cashflow opportunity and build customer loyalty.
If that is not suitable for you, you can send them imprinted jars or
cloths. Lots of sources for those.
You are correct, you don't want to send them to the competition.
Even if what you do cannot be had in their local market, they still
may spend their jewelry dollars elsewhere.
Best of luck