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Hismercy,
I guess you are not welcome hahaha jk. I don't think having that letter makes a whole lot difference. Most important thing is the Plastic which you already have. If you have noticed, Your plastic card was attached to a letter which serves the same purpose. If you read that letter carefully, it reads the same information which a Welcome letter reads minus the WELCOME part. As it says "Save it for future inquiries you might have with USCIS" (which i hope u don't). Thanks