5. The speakers's problem is that they should buy the a gift quickly for their friend who plans to leave soon as quickly as possible.
The man and the woman suggest two possible solutions to the problem. The first suggestion is to go to the department store today and buy the gift. The second suggestion is to mail the gift to their friend later.
I think the second suggestion is better for them. This is because they have no time to visit the department store today because their schedules are already full of with classes. Missing classes to buy the gift (Awkward sentence) is not a good sensible idea. Also even if they send their gift to their friend later, they can express their mind (What minds exactly? Explain more about it) enough by packing the gift with their sincere letters.
For these reasons, I would suggest the second option for them .
* Full 로 가실 때에는 전치사 of가 적합합니다.
또한 위에서 제시된 어색한 문장의 경우, Buying gifts instead of attending class로
전환해주시는 것이 좋습니다.