Changing Shipping to print a label

[sherrylovestreasures]

So, I just made my first ebay sale, YAY!!!  But the buyer asked if I could change the shipping method to Priority 1-4 days, and I said, "Of course!" Now, I'm totally lost.  

1. I didn't see that option on the shipping label printing page, but I did see a bunch of Priority 1-3 day options, so I'm guessing that's pretty much the same deal, right?

2. Next, he's already paid for shipping one way, and I need to give him some money back because of the method of shipment.  How can I accomplish this? Is there a way to do it when I print the label, or do I have to give him a refund through PayPal?  And if it's a refund, how do I do that?

3. I want to choose Priority 1-3 day large package.  It says in parenthesis under that choice that my package can be up to 70 lbs for the same price.  Mine was 1 lb. 3 oz, and $8 and some change.  When I cut some bubble wrap off and got it down to 15 oz, the price changed to $6.20 with my discount.  I tried changing the weight to 50 lbs. to see what would happen, and the shipping went up to $50+.  I don't understand how to properly put in the shipping I guess.  If it should be the same price no matter the weight, why does it change when I change the weight?

4. Can I use a Medium flat rate box for the Priority 1-3 day large package option?

Thanks in advance for getting rid of my stress!!!

Sherry