To set a price:

Press and hold the item — select “Set price” — input your desired price — tap OK. 
 
To edit a price:

Go to the desired item — tap the price field next to it — make the necessary adjustments.
 
If you wish to keep your significant shopping partner updated on the new offers simply head on to the FAQ section detailing the Synchronization of lists and make sure you are on the same page. Literally 😊