Why does my credit card statement show multiple charges for Pressbooks after I've purchased an upgrade?

It's likely that the charges to your card were a protocol of the transaction service we use, Stripe. According to Stripe: 

"When storing customer details or using Checkout to collect payment data, we send over a request to the issuing bank for either a $0 or a $1 authorization to verify that the card is issued and the bank will allow it to be authorized." 

To read more about this process, you can visit https://support.stripe.com/questions/why-does-my-customer-see-an-extra-1-00-charge-on-their-statement