A "received, approved and finalised" affidavit between parties is not binding

My recent interaction with Clientele legal informed me of my misunderstanding of common commercial terms: 1. When suppliers provide a quote/pro-forma invoice the description of goods/service and payment terms are not binding but e-mail/website information is binding. Please note the quote/pro-forma invoice did not make any reference to e-mail/website information. 2. If the buyer provides the supplier with an affidavit confirming payment amount, payment terms as "full and final" payment and a detailed description of goods/services to be provided and the seller accepts the affidavit with the words "received, approved and finalised" and also accepts the payment for such service the seller still has the right to change the amount payable for the same service/goods while the buyer has no rights. An affidavit which has been accepted by both parties is not legally binding and should a dispute arise has no value whereas selected e-mail correspondence (to a related third party) is binding. Please note all e-mail correspondence was prior to the date of the affidavit. The affidavit reflected the description of goods and services to be provided and the amount payable exactly as per the quote/pro-forma invoice received from the supplier. A year after accepting the affidavit and the "full & final" payment the supplier now indicated that the quote/pro-forma was in actual fact for only 50% of the service as per an e-mail sent to a third party related to the buyer (did not send it to the buyer and was never included in the price negotiation with the buyer).

Clientele legal confirmed the supplier is acting within their rights (as they communicated by e-mail to a third party prior to accepting the "full & final payment" affidavit) and as buyer I have no rights to expect the supplier to perform according to their own quote/pro-forma invoice to me and I also don't have any rights through my affidavit which was "received, approved and finalised" by the supplier.