Ok, quite a few errors were discovered in the last V2.5.4 release and it didn't help that there was an error when the beta was first released resulting in some people getting some old files, so here's V2.5.5 to correct all that.

Changed Files
Here's a list of changed files for those who don't want to upload all the files again

affiliates.php
order.php
viewinvoice.php

includes/processinvoices.php
includes/customfieldfunctions.php
includes/ccfunctions.php

admin/configproducts.php
admin/index.php
admin/configaddons.php

install/install.sql

templates/WHMCSDefault/affiliates.tpl
templates/WHMCSDefault/order-stepone.tpl
templates/WHMCSDefault/viewinvoice.tpl

lang/English.txt

Database Changes
ALTER TABLE `tblhosting` ADD `regdate` DATE NOT NULL AFTER `orderid` ;
INSERT INTO `tblconfiguration` VALUES ('RunScriptonCheckOut', '');