Your word is your word, and on a site like eBay, it's worth more than your PayPal balance. If you tell someone you are sending the kicks UPS the day after you get the payment, then make sure you stick to the script. Some sneaker freaks grab shoes for specific occasions and they might have a deadline of when they need them in. By you delaying a day or two it might mess the whole deal up and you'll get hit with the dreaded negative feedback — which will affect your bottom line. Keep to your shipping guidelines, leave appropriate feedback in a timely fashion, and keep moving towards that Power Seller status.