The seller chooses the method of shipping.
The seller chooses the method of shipping to give herself protection against common problems like complaints of slow delivery or loss in the mail.
Shipping choices are a form of Seller Protection.
The seller cannot charge more for shipping than she advertises.
The buyer does not have to agree to pay more for shipping than was advertised when he bought.
If you are the seller, and the shipping service you want to use (tracked instead of letterpost, or Air Mail instead of Surface) costs more than the one you advertised, you can use it, but the cost comes out of your pocket.
If you are the buyer, and you want to upgrade the service (ExpressPost rather than Regular Parcel for example) you can ask the seller, but he doesn't have to agree. If he does agree, he is allowed to charge you the difference in the cost.
This can be done with a Paypal Send Money request.
If you are the buyer and you ask for a cheaper method of shipping, the seller does not have to agree to give up her chosen level of protection against unwarranted claims.