I sent my passport for cancellation on May 8 th with prepaid priority mail ($16.25 postage) enclosed. I received my cancelled passport today in that priority mail.
Here is what the SFO consualte says about resvised postage from May 15 2007
Since USPS is revising postage from May 15, 2007 all applications for passport being mailed from now should have stamps with revised fees (eg $16.25 for express mail) . As passports are likely to be returned after 15th May 2007.
Applications with incorrect stamp will not despatched after 15th May 2007
If you have already sent it with 14.40 and expecting it next week, I would suggest you to email them and find out what you should do. They are pretty quick in responding to emails.