The offered certificate should be supported by all famous browsers. If the provider can’t guarantee this, then it should be an instant red flag. You don’t want to lose visitors, simply because your website can’t be accessed securely from their favourite browser.
They should have been in business for at least 5 years. The most famous certification authorities have existed for many decades now. You may be getting a good deal from a new entrant, but don’t trade-off experience and reliability for some savings.
Dedicated support. If you ever need technical assistance, the support staff should provide it instantly.
Multiple certificate options. A provider should be offering multiple certificate options or giving you the ability to place a customized order. It’s usually smart to avoid providers offering a one-fits-all solution.
Support for automatic renewals. Every certificate comes with an expiry date. However, before one expires, it should automatically get renewed by the provider.