First, you don't BUY parts through PCPP, you only FIND where it is cheaper to buy a specific part AT. Then you order through THAT place. You don't actually buy anything from or through PCPP.
Regarding the weight of the components, that really is meaningless. The shipping charged won't be by weight, it will be whatever the vendor you are buying from SAYS it will be, based on shipping contracts they have with various shippers. Sometimes Amazon uses the US postal service. Other times they will use Fedex or UPS. It's up to them, not you, unless you pay for expedited shipping in which case it will likely be UPS or Fedex but it will STILL be Amazon that determines the price based on the item, not the weight. Weight shipments are generally only used for private parties or 3rd party vendors that don't have shipping agreements with delivery services.
If you had to pay by weight for every component, especially cases, you'd pay much more than what they are likely to charge you in shipping. Find the parts you want, and it will SAY right on the product page what the shipping charges are unless you are overseas in which case you should try to find parts through an online service that delivers to your region or has a presence there. Paying shipping on a complete build and trying to bring it from overseas will result in you paying more for shipping than most of the parts cost.