I don't have a problem with the concept of buying software on SD card, but hre are my thoughts:
Can you offer SD cards cheaper than the largest online retailers? If not, I'm likely to be paying more than if I download your app and put it onto my own SD card.
What happens when someone has, or says they have, a faulty card?
Distance selling regulations could be a major bummer.
With digital distribution, you get the cash quicker, the customer gets the product quicker.
On a similar note, people could probably get a pirated copy far quicker than your SD copy.
What about updates? Have I got to buy another card?
Digital distribution enables scaled down trial versions.
With physical distribution, you are creating a level of human labour required for every sale. Digital distribution could be fully automated.
I could probably go on. Just embrace the new digital age. You will be better placed for future developments.
Looking forward to seeing your software though. You have any teasers / videos etc of existing stuff?