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Once purchase order is issued, it is a kind of difficulty to again negotiate the prices, however if you get some kind of information from other competitors about lower prices, then you could ask for some kind of discount such as asking for bonus quantities etc.
There are many way to negotiate after ordering but it's depend on your negotiation reason like that1=Change of Fashion
2=Change of Style
3=Change of Weather
4=Change of Quantity
etc
It is common protocol that once PO has been issued, no negotiation for discount will be entertained, unless both parties shall gain for the negotiated discount.
In general, price should not be negotiated unless there are solid and cogent reason to take such an action. Such reasons may include drastic decrease in prices of raw material, significant changes in tax structure applicable on raw material and/or finished product, etc.
If you are able to increase the quantities, you can negotiate the prices.