If you want to set auto-renewal by default for the future hosting purchases, go to Account > Profile > Billing, then click the Edit button in front of Auto-Renewal:
For auto-renewals, we'll attempt to charge your Namecheap account balance first. If there are insufficient funds in your account, we'll try using your payment cards next – default card first, followed by any alternative cards, in the order you've specified.
At this step, you need to enable auto-renewal for hosting and other desired services in the list and click Save Changes.
NOTE: We won't be able to auto-renew if sufficient funds are not available via one of these payment methods when the product or service expires. Any settings changes you make here will be reflected next time you check out.
NOTE: Please make sure that the "Use for auto-renewal" option is chosen, otherwise the auto-renewal will not happen. You can find the detailed instructions in the "Can I use a credit card to set up auto-renewal?" article.
Currently, PayPal cannot be used to pay for automatical renewals of your services.