I have purchased Expression Engine 3.4.2 and Expresso Store 2. However I can't get the addon to be shown in the add-on section with Expression Engine. After uploading it to several folders I started looking into the release notes of Expresso Store 2 and noticed that there are no notes about Expression Engine 3.4.2. Is it true that Expresso Store 2 is not compatible with Expression Engine 3.4.2?
Looking on Devot-ee it looks like it's only supported by EE2. If it told you to install it using the /system/third_party way then it's definitely EE2 only. EE3 plugins are installed in a different directory. You may want to look at CartThrob. Most plugin vendors offer a 30 day refund if it doesn't meet your needs, so you may want to look into that.