bonjour
1- I would get a card for a bookstore (at the shopping center)
Je voudrais acheter une carte-cadeau pour la librairie au centre commercial .
what it really means = I would like to buy a gift card for the bookstore at the shopping center ....... I didn't get what you were exactly asking for ....
2- They would have to go to the bookstore, the jewelry store, and flower shop.
Ils devraient aller à la librairie, à la bijouterie et chez le fleuriste.
3- A good amount to spend would have to be a few hundred dollars.
Un montant correct à dépenser serait de quelques centaines de dollars.
OR better ==>
Quelques centaines de dollars serait un montant correct pour des dépenses.
4- I decide what present I am going to get for a person by thinking about what would be important for them.
Je décide quel cadeau (je vais acheter =) acheter pour une personne en pensant à ce qui serait important pour elle.
you don't need to write "je vais acheter", this makes the sentence too 'heavy'.. "quel cadeau acheter" is enough and means the same..
by thinking about = en pensant à
hope this helps ☺☺☺