Simple. A week or two after the campaign ends, I will send out a backer survey that asks a question regarding how you want your pledge money to be spent.
If, for example, you pledged $11, you could say that in addition to the $1 base tier for the full game, you'd also like to have:
Your name in the credits (-$2)
A digital copy of the original soundtrack (-$3)
Early/community access (-$5)
Or you could simply choose to:
Name a weapon (-$10)
The choice is yours. Either way, they both add up exactly to $11, the total amount of your pledge.