First I would like to thank you for your excellent work and the effort you put into the documentation for us makers!
I'm also new to this plattform and registered especially for your project.
I ordered the rails at Blurolls shop too and am having an issue with 1 rail (not the carriage) which prohibits that the carriage is running smooth on 1/3 of the rail. The communication with Blurolls is not good: First they told me that the rails+carriage were of the wrong type because they supposedly got "the ones for heavy duty". I then contacted Hiwin here in Germany and they told me that they do not at all offer different types of MGN12H. Now I'm in the process to get a refund and/or to exchange the faulty rail which is more complicated than I hoped and as I experienced with other dealers on Aliexpress in the past.
If you do not trust me because I'm new to thingiverse I could mail you further information if you want me to.