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Adding custom funds to steam is a pretty simple thing that not many people are aware of, of course the amount you add needs to be over the steam minimum amount (UK £4 - US 5$)
Not sure if its still 5$ in the US.

Requirements
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Go to the steam page for adding funds: You must login or register to view this content. then press CTRL + Shift + J to open the JavaScript Console, In the console you want to enter "javascript:submitAddFunds( AmountHere );" and replace AmountHere with the custom amount you want to add so for instance i want to add £6.00 which will be 600 as you can see in the image below
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Once you've done that it will Re-direct you to the payment method screen, from there you go ahead and add the funds like you normally would.

Proof
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That sounds like doing anything on steam during the summer sale sadly.