Honestly the best way to learn is to do it with $20 and see how easy it is. Go through the process on BitInstant.com, and you'll see all the instructions you need.
I tried it. I selected 'walmart' and 'to email' options, and tried depositing $20:
It says I'm going to get $19.20 for my $20. A 4% fee. Seems good:
It asks me for a 4 digit PIN, then tells me I'm going to receive $18.20 for my $20. A 9% fee. What happened there?
Oh, and it also tells me there will be a further 1% fee for the USD to BTC conversion. So that's a nice round 10% fee in total.
Then when I get the voucher to print, it turns out I have to pay $23.95 (a convenience fee of $3.95 has been added):
That makes a total fee of exactly 25%. I pay $23.95 to get $18.202 and then have to pay an extra 1% on top of that for the conversion.
I wonder if there's some way of presenting the true cost up front, and making the fees clearer. It seems, for instance, that I end up getting more money if I deposit $99.99 than if I deposit $100, because the rate goes up 1% at $100. But the only way of finding that out is by trial and error. I'd like to see the fee structure spelled out clearly.
The rate is a flat 3.99% all around. When you do 'Bitcoin to email' coinapult charges an extra 1% for the conversion and trading fee which we clearly display in orange on the next page.
For some transfers, zipzap adds $3.95 fee on it. So yes, when doing $20 it may seem $5 is too hefty, but on $500 its alot less.