[Europe] Mail by post tracking CRM

I'm a crm-noob looking for advice for the following; I've created two different WordPress websites A+B which will be the landing pages for my targets. The idea is to send post mail addresses different mail contents [A+B] with a tracking code which they can use on the website so I can track the different campaigns.

I'm looking for something which can group contacts into different campaigns, have an easy way to use templates to create PDF to print and mail out. It should have an API or something which can work with WordPress forms or any analytics so I can track visitors back to the campaign.

I don't have a lot to spend < 100 USD/mo, but would like to see if there is a good alternative compared to stitching things together with Integromat/Zapier.