I can't speak to the QC issue except to say that I've been getting kits from BYOC for many years and only one (the first one, actually) had a missing or incorrect part. Perhaps it is a result of trying to operate a business in the midst of Washington state's mandated shutdowns. At any rate, I know it's frustrating not to be able to complete the kit right out of the box.
What I can comment on is BYOC's response to missing or damaged parts, which is that they will ship you the correct part(s) free of charge if you let them know there was an error. Email
sales@buildyourownclone.com and let them know what's missing. They'll take care of you, no need to spend more money at Mouser.