I'm having very close contacts with Sampath Bank and I'm clear on this issue. Most of the debit cards they provide are locked for local use like Super Market purchases, etc... You will have to either request the manager to unlock the card or change the card to another type. Due to increase of card frauds, they had increased their security measures that resulted in these things. I have a Debit Card (and most of the other cards from almost every private bank

) and I use it to purchase things online without any problem. Do not keep your trust on forums (including what I say) for these things. Just go and meet the manager of Sampath Bank yourself.
Also, I think Web Card is the easiest thing to use online. You get an online account from Sampath bank. Open a savings account and link all the accounts including Web Card to your online account. So you will be able to transfer whatever amount you want from your savings account to Web Card/Credit Card 24/7/365 over a web interface from home without any need to go to bank. So if you want to buy something at night, you can transfer funds and in few minutes, it is will get transfered and you are ready to place the order with the Web Card. This is something that I use for many years.
Another suggestion is to ask your dad to get a credit card. Since he is a government servant, banks usually offer cards without any condition since they are the most trusted people for them. I only recommend Sampath Bank for high tech things like using the card online, etc...