You should try and visit their office if you want to save some money and ensure that they won’t ask you to resubmit/ or ask for more documents, correction etc. If you have time issues or want to get something done quickly/ or if they don’t have any time slots available for the next month or so, then go ahead with the mailing procedure.
In short, people working at BLS don’t know what they are saying and you’ll always get a different answer when you call them. Personally, I also feel that they get some sort of commission, every time they recommend you to use courier services. Purolator isn’t that expensive if you deal with them directly, it’s damn expensive when you go through BLS for their courier services though.