Marketplace
Confirmation of Steam Summer Sale dates: http://www.techradar.com/news/steam-summer-sale-2017-dates-r…
Sale starts at 6pm BST on 22 June which is 3am on 23 June Australian time (Eastern time).
If you have or create a UK Paypal account and spend 20 pounds Paypal will give you a discount of 25% and deduct 5 pounds sterling.
Whether or not that is worthwhile depends on the prices of the games you want to buy so compare if they are cheaper to buy on the UK store or not - if they are it might be worthwhile making a UK account to get the discount.
Also check whether the keys you buy can be redeemed on an Australian account which will be the case if you buy Global keys.
Read the instructions/terms+conditions to ensure you do everything in the right way and order to get the discount.
I am not sure if this is a targeted offer but since the link came from the tweet I assume it is not.
Discount expires on 5 July 2017!
Note: According to the terms and conditions the offer expires once Paypal has paid 24000 pounds to customers. In other words, the offer/discount is available to the first 4800 people making use of the offer. Not sure if Paypal will actually stick with that or not but this is something you should be aware off. If you want to make use of the offer, do so as early as possible. I assume Paypal will tell you before you make final payment if the offer is still available.
I hope this is useful for somebody.
Enjoy.
Setting up a British paypal isn't worth the effort to save all of $8.