Ticketing deadlines and lack of responsibility
So sorry to hear about your ordeal. I hate to say this but it happens so frequently. I am a Travel Consultant with 26 years in the industry ( Corporate Travel). There are 2 main issues at fault in your situation; the 1st one I am not going to comment on but it looks like the vendor you trusted with your purchase made a mistake and rather than own up and do what is right they hide behind the fact they are an online business that do not care about image or integrity ( I might be wrong).
The 2nd issue is Airline imposed ticketing deadlines for all reservations made. For every route an airline flies they have a fare (price) structure that is unique to the average 26 different fare levels (on a single route !). Each of these fare categories ( also known as farebasis') have their own set of rules and restrictions which cover a rather daunting amount of "must's and can't do's" :
Advance purchase, Day or time of flight, which flight can not be used, Minimum stay, Max stay, blackout dates, change rules, cancellation rules etc. On all a simple flight from Rio to Sao Paulo comes with 16-22 pages of " fare rules, all of it must be adhered to in order for any licensed IATA company to issue the ticket.
One of the clear stipulations attached to any reservation is Advance Purchase "deadline" 15 years ago we, the Travel Agents had the ability to place a phone call to an airline representative, obtain an authorization to process a booking with a missed deadline without any implications to us or the client.
Over time things have changed. For the lowest fares on almost all airlines the imposed ticket deadline is : 48 to 72 hours after the ticket has been reserved OR 7-21 days in advance which ever comes first. Some carriers demand payment by midnight the day of reservation.
The reservation is almost always created via a reservation system (GDS) where the Travel Agent (or online booking outfit) receives the deadline in Date and time of date at the time pricing. IF the individual in charge of selling/ processing (issuing) the ticket by applying payment misses the given deadline, the reservation is not always cancelled by the airline but the reservation can not be priced meaning a new reservation is needed often at a higher price.
This seems to be straight forward: have a system that ensures a ticket (your purchase) does not miss a deadline by setting reminders etc.
Having shared this I need to throw another airline head shaker trend into this : Despite the carrier publishing the purchase terms and enforcing them without mercy, several carriers now "reserves the right" to demand payment for a reserved ticket at ANYTIME regardless of what their standard terms state. This is done typically when a flight is almost sold out, for the purpose of reaching max revenue for a flight. When an "early ticketing" demand is made it is done in form of a message that is appended to the Agents reservation. This message is VERY easy to miss as it is typically only visible by reviewing the reservation in detail.
I, as a Travel Pro who assist a repeat clientele have learned this lesson the hard way. Discovering that a pending reservation quoted at a certain price no longer can be processed but also will result in a hefty increase in fare when re-booked is something I have had to deal with a few times. Every single time I personally covered the increase in cost ensuring the traveler ( client) never knew what went on "behind the scenes".
I hope you are able to secure your ticket without having to pay more than you originally expected......