It is admirable that the vendor has taken on this responsibility. However, by definition, this means the vendor has access to any potential sensitive data on the drives.
Merely deleting the data only wipes out the directory listings for the files. Even performing a format only destroys the previous boot record.
In general, data can be potentially recovered unless the data location is overwritten by something else.
As a minimum, use a secure wipe utility that meets DOD 5220.22-M specs. This spec require that: all addressable location be overwritten by an ASCII character (can be user defined), then overwritten by the characters complement, then overwritten by a random character, then verify the contents of all addressable locations. This is a total of 3 overwrites in addition to the verification. This standard is used by the Department of Defense for sanitizing magnetic media with classified information.
There are numerous software utilities on the market that will meet these requirements (some are freeware, some cost money). Some include even more stringent wipe algorithms (up to 35 wipe passes). Obviously, the greater number of passes, the slower the procedure. Run a search on Google using the keywords "DOD wipe" or "DOD 5220.22".
Hope this helps.