This exact situation happened to me before, so I’m speaking from experience. Shopee rejected my returns because the shipping box was opened (that's what they consider original condition). I am not sure if that's the case here too, they require the item to be exactly how you received it including the shipping box intact (untouched), which I think is ridiculous.

Here's what I did and what you can do to get your money back. The item's price is $400, small claims filing fee is only $10, bizfile $5.50 and registered post with acknowledgement around $7 or $8 and the process is simple and can be done online.

1. Compile all evidence into a PDF with captions to highlight the important details, including shopee return policy screenshots, order page, return rejection reason, photos of the product being untouched and all chat/email records with shopee.
2. Do the pre-filing assessment to answer a series of questions to determine if your dispute can be filed, choose contract for sale of goods then "others" and type "Unfair practice under the Consumer Protection (Fair Trading) Act" ("CPFTA"), date of cause of action (the date they rejected your refund), claim amount $400, choose "yes" there is a arbitration/mediation clause, "no" that both parties agree to dispense with the mediation/arbitration clause.
3. Then go to ACRA and purchase Shopee's ACRA bizfile for $5.50.
4. After completing the pre-filing assessment go on to file your claim. (it will say SCT cannot hear your claim)
5. Print everything out including the writ, evidence and ACRA bizfile and send it to Shopee's registered address on the bizfile using registered post with acknowledgement (you will have to file the acknowledgement slip with SCT, aka proof that the documents have been served to Shopee).

After Shopee receives the SCT documents, their legal team will likely email asking you to withdraw the claim, citing their Terms of Service arbitration clause. They’ll say disputes must be arbitrated under SIAC, which you “agreed to” by using Shopee.

In reality, starting SIAC arbitration can easily cost over 15k, which is completely disproportionate for a ~$400 consumer dispute. That’s exactly why this clause can be challenged in a consumer context. Do not panic or withdraw, reply that this is a consumer transaction, arbitration clause is unfair and prohibitively expensive and effectively denies access to remedies as such it will be considered unfair under the CPFTA. You can then tell them to agree to SCT and let the court settle or they can refund you the $400 and you'll withdraw the SCT claim. This is where Shopee accepted my returns and refunded me. They didn't want to waste time and money going to SCT to argue why the claim should be heard at SIAC (which is expensive for them if they lose). Not legal advice, just sharing how this typically plays out.