Shortly after the campaign has concluded (about 14 days), you'll receive an email from Backerkit! You'll be able to select add-ons through Backerkit, including ones that weren't in your tier.
If you want to prepay for add-ons as part of your pledge, simply pick your pledge level and add the additional cost from the add-ons section to your pledge amount! Think of add-ons as your personal a la carte menu so you can get exactly what you want.
So, if you chose the $50 tier, and wanted to purchase the $20 3D printable minis as well, you'd set your pledge amount to $70.
And that's it!
If you don't do it this way, don't worry. Add-ons will always be available later for purchase through Backerkit.