I remember about 20 years ago sending a cheque in US dollars to BPS Kandy and waited several years for my books to arrive. It was an exercise in patience! I finally plucked up the courage and sent a letter asking what happened whereupon they promptly sent one back telling me that the post office returned the parcel, despite my redirection notice at the post office!
In the last few years I have purchased most of my books via www.dhammabooks.com
in Australia where I've developed a bit of a relationsip with the owners who are good people providing an excellent service. I ordered Gombrich's 'How Buddhism Began' from an Indian clearinghouse and it arrived in record time. While I have not used them, I would not hesitate to also use www.pariyatti.org
or go direct to BPS or other publishers. Occassionally its worthwhile investigating amazon as they do stock out-of-print second-hand books.