I’ve written two plugins for WordPress. They’re both free, designed primarily for authors, and available from the WordPress plugin repository.

Mobile Banner
The Mobile Banner plugin was written to address a client requirement. It adds a discreet banner to the bottom of the screen when viewed on mobile (not on desktop). This banner can have whatever text the users wishes, and can link to a any URL.
The client that it was written for used it to link to their mailing list sign-up page, and it is used the same way on this website.
The plugin is available from the WordPress plugin directory, or you can search for “Mobile Banner Robin Phillips” in the WordPress plugins screen.

Local Links
Authors need to link to their books so that users can buy them. Some online stores have a site for each country that they operate in. It’s not practical to list the link for each country, but users often have to use their own country’s site. Users in the UK, for example, have to buy from amazon.co.uk, not amazon.com.
The Local Links plugin automatically edits links so that they point to the user’s local store. The links are tidied up, to make them simpler and avoid tracking. It will work on existing links and new ones. Simply add links to your site as usual, and they will be rewritten on the fly to send your visitors to their local site.
Stores supported are Amazon, Apple Books, Apple iTunes (audiobooks), Kobo, Alibris, Google Play, and AbeBooks.
Amazon has different affiliate codes for the different country sites. If you have Amazon affiliate codes, the relevant country’s code will be added to Amazon links.
The plugin is available from the WordPress plugin directory, or you can search for “Local Links Robin Phillips” in the WordPress plugins screen.