Deposit ZP

I want to deposit ZP so badly, and i just bought 2 Subway gift card($15 each). But I still being confuse on 2 things:

First, can I combine those $15 gift card together(= $30) to buy ZP and how?

Second of all, if I have $15 gift card and I will buy 10,000ZP, which is $8.99. What happened to other $6.01? Can I use it for my next deposit or they will took it.

Thanks for helping me
P/s: excuses my English

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