@ObiBramKenobi I would contact EA Help at https://help.ea.com/contact-us/?product=ea-play&platform=steam-pc. I would probably either try Topic "Codes & promotions" + Issue "Promotion questions" (the support options I am presented when using that combination are chat and "get a call", but your options may vary), or Topic "Orders" + Issue "Unable to purchase" (the support options I am presented when using that combination are chat and "get a call", but your options may vary), or Topic "Report a bug" + Issue "Give feedback" [the support options I am presented when using that combination are Answers HQ and email (choose email, not Answers HQ), but your options may vary]. Given that the sale ends September 13, I would preference chat or "get a call" over email.
EA Help's current hours of availability are https://help.ea.com/help/faq/temporary-changes-to-support-options/.
I would have all the EA Play documentation you can find in front of you when contacting EA Help:
E.g.
https://tos.ea.com/legalapp/eaplay/en/PC/
https://www.ea.com/ea-play/faq#steam-faq | https://store.steampowered.com/subscriptions/ea (the info on Steam is the same as the FAQ)
If they tell you the 10% discount doesn't combine with a sale discount, I would request that they provide you with a link to the documentation where that is specifically stated. If they provide you with a link, I would review that documentation while they are still chatting with you in case you don't see any obvious notice in that documentation and need to follow up with them for further details. The only notices I see printed in the EA Play Terms are "Member 10% Discounts: excludes Star Wars: The Old Republic, certain partner titles distributed by EA, game time codes and subscriptions on PC; also excludes backwards compatible Xbox 360 titles on Xbox" and the vague "Conditions, limitations and exclusions may apply; availability of member benefits and Play List titles may vary by platform and territory and may change from time to time". The only notice I see printed in the FAQ is "Please note: while EA Play is available on multiple platforms your discount only applies to the platform you joined on".
It's entirely possible the 10% discount doesn't apply to EA sales or this specific sale or to EA sales on Steam, but I haven't encountered that before (maybe the issue is limited to EA Play on Steam or maybe a policy change was made when Origin Access and EA Access became EA Play). Please consider posting to this thread again with any clarifying information you get from EA Help in case someone else runs into the issue you are experiencing.
You could also try Steam Support, in case this is somehow related to Steam's Subscriber Agreement or a glitch on their end. I think Steam Support may point you to EA Help, but at https://help.steampowered.com/, you could try Purchases > I can't complete my purchase on the Steam Store > Game or other software > Search (for The Sims 4) > Products - The Sims 4 > Please provide details about the issue you are having (mention the specific packs you are having the issue with).
