Looking up the prescription regulations, only >2500 IU requires a prescription, so if you can find 2500s to import, that's over-the-counter and can be lawfully imported for 90d personal use. https://www.napra.ca/national-drug-schedules/?_nds_drug_name...
If you try to have 10000s mailed from USA by USPS, there's a very rare chance that it could be reviewed by Health Canada and they would deem it prescription-only and seize it, or they could "use their discretion" and still let it through anyway because seizing is a lot more paperwork.
Don't bother with UPS or Fedex shipping into Canada. Ever.
Note: if you're in Canada on a work/study permit, there are some exemptions allowing you to import non-narcotic prescription medications.
Are you referring to brokerage fees? UPS does not charge brokerage fees if air shipping is used; basically, anything other than Standard qualifies.
I am jealous of Americans and their 10,000IU Vitamin D pills. Canada limits the same to 1,000IU per pill. Would this be allowed to cross the border or no?