I get wrong postcodes now and then, but using MyPost for postage it will pick up the error. At the end of the day, the mail centre will generally fix the problem when it comes to parcels.

Letters are machine sorted using the postcode and that's when your letter can go astray, but if the postcode is within the same mail centre sort at the end of the delivery, it will be fixed as well.

Don't sweat over it.