Disclaimer: I am not a lawyer, just a not-totally-uninformed layman. Legal concepts may vary from one jurisdiction to another, so the following is merely an expression of what, I hope, is informed common-sense!
The practice of trying to increase an already agreed price is very common in the UKhousing market, and is termed "gazumping". The present problem is not of the same order of financial magnitude, but the same legal principles should apply.
I must respectfully disagree with this opinion.
There should be no problem if an invoice has already been issued by the seller at the agreed price. But an invoice is a formal request to the buyer to complete his part of the already existing contractual terms by paying the seller. An invoice is not a contract.
The seller is thus not entitled to redefine the terms of the contract (in this case, price) without the agreement of the other party - the buyer - by introducing new terms and conditions in an invoice.
"Subject to change without notice" is commonly appended to published prices in catalogs, online etc. It means that the price quoted is, in effect, a "tender" price or "offer to treat", which is to say, the prospective purchaser may offer to purchase the goods at the named price, but the seller may still say, before accepting the order, that the price has, in the meantime, been increased.
However, "Subject to change without notice" cannot be taken as a contractual obligation on the part of the buyer to accept any price that the seller may define retrospectively, after the buyer has offered to buy at the stated price and the seller has accepted that offer (=purchase order). If it were otherwise, the seller could retrospectively charge, say a million, a billion, whatever... That is obviously making nonsense of the concept of a contract.
A contract is basically a formalized exchange - typically of goods for money, but of course also of work for money, or work for goods.
It requires the form X for Y, not "anything one party feels like for Y".
For instance, "nothing for Y" is not a contract, it is a gift!
Once the seller has accepted the buyer's offer (=order) at the quoted price, then a contractual situation has been established. Retrospectively wishing to increase the price is thus an attempt by the seller to withdraw from the terms of the contract and the buyer is therefore no longer obliged to adhere to the contract - it is the seller who wishes to withdraw. The buyer may indeed be able to compel the seller to complete the contract at the original price.
All this is not just legal theory, but of eminent practical importance if you are, for instance, purchasing a house.
In the case of the item under discussion, the buyer appears to have the following options:
1) to formally withdraw from the contract on the grounds that the seller is unable or unwilling to adhere to the terms of the contract,
2) to compel the seller to complete the contract.
Option 2) may well be used for a contract as important as house purchase. In the case of an item hardly worth a 3-figure sum, the legal costs swamp the value of the item. I would therefore recommend that, in this case, the buyer uses option 1. In writing, of course! And immediately - before the item is delivered. For the reasons explained above, it is unlikely that the seller will try to enforce a completion of the sale at the higher price.
But, as stated at the outset, I am not a lawyer, and things may be different where you live!