The best for buying multiple domain names at once (costs start at $5.99 per year).

Gandi allows you to register, transfer and renew domain names. It provides an easy-to-use domain search and domain offer tool that lets you choose from over 700 domain zones.

You also get a free SSL certificate, two free mailboxes with 3GB of disk space, and unlimited email redirects and aliases for your domain name.

Although most of the prices for domain renewals are equal to their registration prices, they are still too high. However, you will get special offers at a discount when you register or transfer several domains at once.
Gandi Benefits:

More often than not, domain renewal prices are the same as their registration prices.
You can also sell your existing domains on Gandi.
This registrar allows you one-click connectivity to third-party business applications.
Unlimited user support and personal manager assistance.
This registrar allows you to manage all your domains and other products from a single dashboard on all your devices.

Cost:

Starting prices for some of the domain suffixes:

.us – $15.50 per year.
.com – $15.50 per year.
.fr – $15.50 per year.
.io – $38.00 per year.
.net – $18.50 per year.
.co.uk – $9.88 a year.
.online – $5.99 a year.
.me – $6.50 per year.