The payment is only registered when you revisit the page with the Bitcoin address. I'd guess that you paid, didn't return to the forum for some months, and the price increased in the interim. When I increased the price, there was an automatic grace period which prevented this from happening from eg. one hour to the next, but not over such a long period of time.
If you'd emailed the pbbugs address, I would've accepted the old payment.
ohh... I see. I think I didn't revisit the page immediately after I made my first payment. When I opened it a couple months later, it's showing a new address and total amount reflecting the price increase. Actually even after I made my second payment and reopened (refreshed a few times) the page, I still can't post eventhough the page is not showing payment request anymore. I had to relogin to remove the proxyban warning before I can make my first post. All is good now, thank you