2.x, 3.x, 4.x and 5.x refer to the different versions of Miva Merchant. 2.x means any version 2 of Miva Merchant, so this would include: 2.0, 2.13, 2.24, 2.25, etc. The version number here indicates which versions of Miva Merchant our module works with. If a module does not show an entry for the series of Miva Merchant you have, it will not work with your version. Please contact us to check.
If a module is not listed for 5.x, it will not work in Miva Merchant 5. All 5.x modules work in all versions of Merchant 5, including Merchant 5.5
Viking Coders tries to keep all our module upgrades free. Only when a module requires substantial rewriting will we charge for an upgrade. For example, we have kept upgrades from version 3.x modules to 4.x modules free as there were few changes in the Merchant API and it was relatively simple to rewrite the modules so that they work in all recent versions of Merchant.