How can I convince my parents!!!?

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How can I convince my mother to use her credit card? I want to buy a game from Steam, but I don't have a debit or credit card. I've offered to compensate for the game and the taxes, and do the dishes for a month. She thinks that her PIN number will be stolen if she uses it on the internet. I've explained to her that Steam and PayPal are both completely free, but she only gets angry and tells me that I'm ruining her day. This is the first time I've asked her so no, I'm not a spoiled brat or anything of the sort. Actually she's perfectly happy using her credit card over the phone to buy clothing, but when it to comes to the internet she avoids it like the plague!!! What do I have to do? Up to this point I've paid for most of my PC games over the counter in cash! But they don't sell the game I'm looking at in any stores aside from the digital ones. Must I make a power point to convince her? An article? Should I get someone else to explain that it is safe!!!? HELP ME! T_T