Two-colour letterpressed goodness

As part of the 10-Day Letter Challenge I received this wonderful diminutive letterpressed note from Robin Reich of Reich Paper. Designed and letterpressed by Benjamin Paul. Robin and I are both dog lovers so it did not escape me that she put canine postage on the envelope but once again I’m confused about the fact that there are no numbers on these stamps. How do you know how much postage you are putting on an envelope if there are no numbers on the stamps? I really need to get to the bottom of this.