There are two ways to add an authorized user to your account – online or by phone.  

PayPal Cashback Mastercard account holders:

• To add a user online, access the servicing site by logging into your PayPal account and clicking PayPal Cashback Mastercard from your account dashboard. From here, select Manage and click Manage authorized users and then Add authorized users. To add a user by phone, just call the toll-free number on the back of your card or use the phone number listed below.
• Contact us at: PayPal Cashback Mastercard - (855) 938-3684, 24 hours/day, 7 days/week    

For eBay Extras Mastercard account holders:

• To add a user online, log in to your account online by clicking your eBay Extras Mastercard from the account dashboard. Once you’re transferred to the Synchrony Bank servicing site, select Manage Cardholders from the Services menu, and click Add a New Cardholder at the bottom of the page. To add a user by phone, just call the toll-free number on the back of your card or use the phone number listed below.
• Contact us at:  eBay Extras Mastercard: 866 419 4095, 24 hours/day, 7 days/week  

There's no charge for additional cards. You will be liable for all transactions made on your credit card account by any authorized users as if they were your own transactions.