********* behavior from pick and pay online !!!

I ordered online with pick and pay for future delivery. a Couple of days before delivery date I realised I have to change my payment details to pay from another credit card. After changing my card to another saved card on the system, I received confirmation that the order was updated. To my surprise I noticed that the order is now more than 400 rand more expensive than it was originally!!! This due to some special prices I have been originally charged has now changed to the normal price. Nowhere on the system does it indicate that if you change payment method that specials falls away and if this is the case why only some items? And it can not be that they are charging me the current price as one of the items is now actually on special for CHEAPER than my original amount but I am still charged the FULL amount on the new invoice. This is not the first time it has happened. The last time I did not keep my original mail for the order but this time I did. I think it is ********* behavior and I am sure they get away with this a lot with other shoppers not comparing prices. I will do my utmost best to let everyone know how ********* pick and pay is in their business dealing with clients. Not happy at all!!