Place formalin container wrapped in a paper towel in a ziploc bag and place in a small box or padded envelope. Tape around the top of the container and bubble wrap around it give added protection but are usually not necessary.
Regular US mail is cheapest and generally fast enough. My understanding is that 10% buffered formalin is not considered hazardous as opposed to the concentrated 37%. FedEx and UPS work also.
Place slide in either a cardboard or plastic slide mailer and then in a padded envelope. A cardboard mailer in a regular envelope is not adequate - they arrive smashed every time!