Ordering meds online

How, how do you go about ordering meds online? Do you get the IVF clinic to send you the prescription or do you they send it to a pharmacy? My us based IVF clinic told me I need to go to a US pharmacy ( I live in Canada) to pick up my meds because of something to do with converting the quantities into the metric system that we use in Canada . It doesn’t really make sense to me since some of you are buying your meds online and in the UK. Thanks for your help!