I want to purchase software for my Ubuntu 12.04 (through Sofware Center) but now I can't use my Ubuntu computer (it's not at home), I'm only with my Mac, and the software I want to buy has a reduced price just until November 3... How can I buy from Ubuntu Software Center from my Mac? I have an account already.
Your purchase is linked to your account, so really you just need to log in "somewhere" and make the purchase.
I recommend using a Live CD or USB stick to do what you want.
You could also boot using parallels or similar.
https://apps.ubuntu.com/ is as close as I can find to an online store and it doesn't appear to allow purchases.
https://apps.ubuntu.com/cat/tos/ - says you can re-download forever.
The only way I can think of to do this is to install Ubuntu on a virtual machine within OSX and download the software on that. As long as you log in with the same credentials on both instances of Ubuntu, your purchase will be remembered.