Last but certainly not least is Selectize.js. This very unique plugin lets you transform select menus into text boxes for selecting multiple items at once.
You can see a few live demos on the plugin page using many different styles. This plugin supports option headers and even subtitles beneath select fields. With the “remote fetch” option you can even pull data from an external API and load those items into the select field.
This entire plugin is totally free and available to download from GitHub. I don’t think it’ll fit into every checkout form but it certainly offers a unique value for larger select fields.
With all these plugins it can be tough knowing what to use. My suggestion is to try a bunch and see what you like best.
But remember that each plugin adds size to your page, so try to stay slim and only use plugins that truly improve your form’s performance & the user experience.