HGTSNS

Items purchased on the 1st of January for $2000+.

Same items on the 2nd of January had the HGTSNS discount valued at $200.

Is it possible to obtain this discount post payment in this circumstance?

You could ask the seller, if it was their discount, but it's doubtful if they would give you a retrospective discount similarly if it was an eBay discount.

 

Edit. I see that it was an eBay discount voucher, so you could try asking eBay CS reps for the discount, but I suspect you'll be out of luck.

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"Start me up I'll never stop......"

It's actually only $100.00, still nothing to be sneezed at.  But you purchased outside the offer period,   so no

 

Items listed on eBay.com.au where a link to these terms and conditions and the coupon redemption code HGTSNS is found in the item listing.Save: $10 when you spend between $100 - $199 in one transaction; Save $20 when you spend between $200 - $499 in one transaction; Save $50 when you spend between $500 - $999 in one transaction; Save $100 when you spend $1,000 or more in one transaction.
  1. Offer Period. This offer commences at 10:00am AEDT on 2nd January 2024 and is available for a limited time only, as long as it is advertised on the eBay.com.au site (“Offer Period”). eBay reserves the right to cancel the offer at any time.

No.

 

It would be lovely for buyers if discounts with specific T&Cs (including starting date) could apply retrospectively, but… sadly not.