From the Hong Kong Sale of Goods Ordinance:
(3) Where the seller sells goods in the course of a business and the buyer,
expressly or by implication, makes known to the seller any particular purpose
for which the goods are being bought, there is an implied condition that the
goods supplied under the contract are reasonably fit for that purpose, whether
or not that is a purpose for which such goods are commonly supplied, except
where the circumstances show that the buyer does not rely, or that it is
unreasonable for him to rely, on the seller's skill or judgment.
Breach of a condition gives rise to the right to treat the contract as repudiated.
In other words, the goods have to be fit for the purpose for which they are sold. I think you'd have a good argument that carrying a bag is an entirely predictable purpose for the shoulder of a shirt ;-) . If they are not, you can repudiate - i.e. you return the shirt and they have to give you your money back. My reading is that this is your statutory right - incidentally, exactly the same as your statutory right under English law. It's just that many stores in the UK also have discretionary policies of allowing returns and exchanges where a customer just changes their mind, it's a commercial decision for them, not something the law obliges them to do.