Yea, hopefully they are not opened. Even if they are I bet they come in packages of 10 - they do sell those 10 back so I'm sure they could figure out some way to swing taking most back??
BTW your education of this is going light speed so if you feel a little overwhelmed by the knowledge that's to be expected. Just pull up your chair to the fire hose and have a gulp. ;-)
Ask wal mart for:
U-100 31 Gauge 3/10cc 5/16 inch Needle 1/2 Unit Markings
or
U-100 30 Gauge 3/10cc 5/16 inch Needle 1/2 Unit Markings
Print out and take with you if necessary.