Slackware: I donated the cost of the DVDs plus postage. Seemed silly to shift atoms across an ocean when the bits can trickle down the phone line from a mirror server about 60 miles away.

Are there any strange tax or accounting advantages for the organisation I'm supporting if I buy actual stuff rather than donating?

I don't know about Canadian tax law, but in the US there is no benefit to earned revenue over contributed revenue, and often having a high number of individual donors can be advantageous for granting reasons.

I've seen some strange things related to exports (primarily because the law seems to be moving at a glacial speed compared to reality). So, e.g. a product exported to another country may be exempt from local VAT (so you can "zero rate" it, hence the checkbox that sometimes appears on order pages). This gets a bit blurry when it comes to digital software sales - is it a product you are shipping to another country? Or is it a service you are rendering in your own country (and thus, not exempt from VAT)?

Not necessarily better but perhaps easier. For example your employer may not have an issue with an expense charge for software CDs but might have a problem with a paypal payment to "deraadt"