One thing that has always frustrated me with Paypal is that while you can download a .csv file for all your transactions you can't do so for just a subset of them. For example I can filter my transactions to show only shipping but there's no way to download just that information. I found a work around.
Not to long ago Etsy put up some of their Seller Deals now most I don't care about but they had a link to a free online small business accounting tool called Outright. Now I do all my bookkeeping using Open Office and Mint so another tool wasn't much use to me but I wanted to check it out to see if maybe it would be useful. Honestly I'm not impressed with it's abilities as an accounting tool, you can't batch apply labels to transactions for example and it doesn't do it automatically for obvious one like USPS or Google Adwords like Mint does.
What it does do is be able to pull ALL your paypal transactions (many other accounting tools like Mint only pull a few months) and if you go to Reports you can select a specific transaction source like USPS set a date range and download a nice .csv for use in your spreadsheets.