I did it and I always get my money.

1. Select your country and select how much Bd you want
2. Tip in your phone number
3. Wait for their message with the code ( maybe they first sent you a message where they ask if you really want to buy it, then you must sent them a message with 'yes' )
4. Tip in the code

That should be all.

A lot of people have issues with it. If you're able to use Paypal, I recommend it over all of the other methods.

Does it work like this (for example) -

I have $15 credit on my phone i buy the $5 package then im left with $10 credit on my phone.

does it just subtract the money from your credit?