I always reply with "sure thing", the cashier will say "that'll be fifty dollars" so ill say "sure thing", instead of what i want to say like "did you know that cash registers were invented in the eitheen hundreds, and no it wasnt jonny cash machine" 'boom tish', ive had some awkward cash register moments in my time as well, but since those days i just use small-talk to kill the time while waiting for the reciept to come through, also your a girl so try complimenting them regarding an item of clothing or hair style, hmmm maybe thats too forward.....you'll think of something but remember that the only reason a cashier turns up to work is because of the money and because if they dont they'll be fired, im not trying to offend cashiers but its not exactly a dream job, its like by job at the bistro, i dont exacly enjoy working there but i know the pay makes me roll around like a puppy dog, thats a funny comparison..maybe jump around like a monkey, anyways so they are payed to consult the customer and show interest, also your not uneducated because your at University and youve done well to get into that realm of education, so pat yourself on the back, and whats it to them if your shy, you dont know them personally so remember that next time your at a shop, that you have nothing to lose even if you think you sound silly and uneducated it will boost your confidence to engage them with small-talk, but also there is nothing wrong with saying nothing either, your there to browse around for items right, also try and relax yourself before you walk into a shop, think of a greeting in your head before you enter