Why are you two saying conflicting things?
How often is it that someone does not have the certificate that I have? Will companies that I buy PC items from have the certificates?
It depends. I would suspect that they might. Its mostly automated whether or not encryption is added or not, if they have your certificate and if they have one, then yes, they will use that. If you don't have one and they have one, then it will not be encrypted. If you have one and they don't, then it wont be encrypted.