Hope you booked it then! Even though that price is no longer available, for the dates you are looking at, do you still get a lower price on the QF website compared to Avis website?
I just checked July 24 - July 31 and the price was the same on both (about $221).
Having said that, there are two different types of discount codes for Avis (and Budget, Hertz and recently Europcar too), which have separate/different codes that affect rates differently. Both can be entered together to 'double up' the discount. For Avis, there is:
1. Avis Worldwide Discount Code (AWD) - effectively a corporate/contract rate/discount (rate negotiated/contracted between Avis and third party, in this case QF - can be either a fixed rate, which can be higher of lower than the standard rate, or a % discount, as with QF) and can change the excess and other associated costs like excess reduction and excess km on limited km cars such as Caprice. Qantas has one, which can sometimes lower the price, but not always. It usually doesn't lower already heavily discounted rates such as the ones we're discussing, but sometimes it does. When it does discount the rate, it's usually by 5% off the base rate (and consequently lowers GST, admin fee and airport surcharge, as they are % of base rate, so actual discount is more like 7-8% from somewhere like SYD), but can be more or less than 5%. It also discounts the excess km charge by the same %, if applicable. The QF AWD would be used for all rates on the QF website, but it is hidden/not shown. I suspect the hidden QF AWD was what was giving you a lower rate than on the Avis website.
2. Coupon discount - promotion codes that discount the base rate only, usually by either a fixed amount (as in this case), or a number of days base rate discount. Remember that actual discount is higher once GST, etc applied. Coupon codes do not generally affect excess or other charges such as excess km.
Budget works exactly the same way (they are owned by the same company and use the same computer system), though Avis coupon codes tend to be more generous and Budget Corporate Discount (BCD, same as AWD) for QF tends to apply more consistently.
Hertz and Europcar work somewhat differently.
Hertz Corporate Discount Program (CDP, similar to AWD) and Promotional Coupon (PC) Codes often no longer work together - when the PC code is entered it will often remove the CDP discount.
Europcar - until recently (when website was updated to current layout) only had one discount code, a promotion/contract code. As the name suggests, it served a dual purpose - either as a contract code which applies a contract rate (rate/discount negotiated/contracted, similar to AWD or CDP) and may change the excess, excess reduction cost, excess km. cost; or a promotion code, giving you a % discount on the rate only, but not affecting excess, etc. Promotional codes are the ones shown on their website. Unlike Avis and Hertz, Europcar do not seem to discount location and other charges with promo codes, so the actual discount is less than you think it might be, rather than more. I suspect this is a function of Europcar generally only offering % discounts, but I have never been able to figure it out and Europcar no longer disclose the base rate and breakdown of taxes and charges in a quote, so it makes it very to work out! Also unlike all the others, Europcar often has promo codes that discount excess reduction. When Europcar updated their website recently, they introduced a coupon code field at the very end of the booking process (contract/promo code is entered at start of booking and both codes are entered at the start for Avis, Budget and Hertz). There was no mention of this new field in any correspondence, but I suspected it would work the same as Avis/Bidget and Hertz do and sure enough, a few days ago, I received an email for $20 off in NZ, which upon checking, applied on top of contract/promo discount. On the brief look I had, it did not appear to end up being quite $20 though (similar to the way contract/promo discounts work), but I can't remember whether it was quoting in AUD or NZD, so it may have just been due to currency exchange.
Thrifty only allows one discount, as did Europcar until very recently. Can be % off, fixed rate, or days off.
The above are my observations gleaned from many years of car rental.
