My question is, even if you load BTC to your smartcard's address without connecting it to your computer somehow, how does the card know how much is on it?
Obviously it doesn't in that case. What I'm proposing is that you stick your card into a machine and deposit your $5, the machine sends a +1 BTC transaction, it gets displayed on the card, and you okay it. The card keeps track of your balance.
For the cards I posted at least, if you browse the site you can see they have cards with keypads as well. It would be possible to manually set your balance with those.
More importantly, how do you set your pin without a central authority doing it for you?
Same solution, cards with keypads. But a pin is not necessarily a requirement, especially when using multisig keys as mentioned. Ultimately a reader is only like $15 anyways so I think your minimum outlay is going to be around $20 regardless of which type of card you use.
I think it's also worth noting that if a card continuously re-uses a single address, then it kinda kills your privacy too.
The high end cards can store several keys, several hundred even.