"I want to buy a bag of rice."
I looked up all of the vocabulary before beginning the lesson and "袋", one of the new words listed for this lesson, is also a classifier for "bag" according to Wiktionary: https://en.wiktionary.org/wiki/%E8%A2%8B. (That source uses "一袋米" as an example.)
My answer, which used "袋" instead of "包", was nevertheless rejected. Seems like it should be accepted.
"Should like to" is so utterly archaic in English that I don't even know for sure what it means. I only know it from movies made before I was born, and I'm fifty.
But I think "should" in that sense means the same as "would" as most people speak English today.
There is a subtle difference between should and would that has been eroded over the past 60 years at least. In this case the difference is important. It is a way to show the difference in meaning between 想买 and 想要买. In one case you want to buy, in the other your desire to buy is somewhat conditional.
I should make myself clearer.
想要买 means you desire to buy the thing but it only indicates that you desire it. Perhaps you won't or cannot buy it yet. It is your ability to obtain that which you desire that is conditional.
想买 there is no implication of conditionality. You want to buy (and you can).