What are the advantages and disadvantages of sending things by air? What might cause delays in transportation?

Salesman: Good afternoon, madam. How may I help you?
Customer: Good afternoon. I’d like to ship some possessions to Canada and was hoping you could answer a few of my questions.
Salesman: Certainly. Please take a seat. Where exactly are you going to ship the goods?
Customer: To Toronto. I’m taking a job there at the end of next month.
Salesman: Right. We can ship your possessions by air or by sea and land. Shipping by air is faster but is obviously more expensive.
Customer: I understand. That’s what I expected. If I want the things there by the 25th of next month, do I still have time to send them by sea?
Salesman: I’ll just check the schedules. ...Yes, if your possessions start their journey by the 26th of this month at the latest, they’ll be in Toronto by the 25th of next month. However, I would recommend that you send them earlier if possible because there are sometimes delays due to logistics or weather conditions.
Customer: Yes, of course. I was thinking of sending them on the 20th or thereabouts.
Salesman: That’s great. There’s a cargo ship leaving on the 21st. Would it be possible to send your possessions on the 19th? It can take two days to get the items to the port and to clear customs at this end.
Customer: Yes, I guess that would be OK.
Salesman: In that case, your possessions should arrive on or around the 19th of next month, though, as I said, there are sometimes delays. Is that OK for you?
Customer: That’s perfect! That’s the very day I arrive in Canada. It could be a bit hectic though. Anyway, I won’t need my things immediately, so a short delay won’t cause any problems.
Salesman: Don’t worry. You won’t be able to take delivery of your possessions that day because of customs checks. Those usually take a couple of days. Will you be packing everything yourself or would you like us to take care of that for you?
Customer: I’d prefer to have your employees do it. As you can imagine, I’m rather busy at the moment.
Salesman: And you’ll want us to arrange delivery to your new home in Canada rather than leaving the items at the warehouse in Toronto, right?
Customer: Yes, please. I know that door-to-door delivery costs more, but it’s so much more convenient.
Salesman: OK then. Here’s a price list. The prices include tax and insurance. As you can see, the price depends mainly on the dimensions of the crates and the number of crates rather than their weight. However, we do add an extra charge for particularly heavy items. Do you intend to ship any furniture?
Customer: No, nothing like that--just books, clothes, a few small electrical items and a few ornaments.
Salesman: If there are any fragile items, such as glass ornaments, I can make a note for our packers. There’s no extra charge for such items.
Customer: Thank you. There is a little glassware. Using plenty of bubble wrap should ensure it arrives intact. Would you recommend three of the smaller sized crates or one of the very large ones?
Salesman: For ease of handling we normally suggest that customers use the smaller sized ones. The large ones appear cheaper, but if they exceed a certain weight they incur an extra charge. We generally only use them for large lightweight items or furniture.
Customer: I see. I think that three of the smaller crates measuring two cubic metres should do the trick.
Salesman: OK. In that case, the total price will be 360 pounds. Is that OK?
Customer: Yes, that’s fine. Thank you. Can I pay by credit card? I have Master Express.
Salesman: Of course. I need to take down a few details. Could I take your name, please?
Customer: My given name is Debra--D E B R A--and my surname is Whealing--W H E A L I N G.
Salesman: And your address?
Customer: 14 Hopton Close. The postcode is PT6 2 JA
Salesman: And the address where we should ship your possession to in Canada?!
Customer: 42a Lake View Road in the district of Victoria.
Salesman: Is "Lake View" written as one word or two?
Customer: Two. The postcode is TR12 3CN.
Salesman: Do you have a mobile phone?
Customer: I do. The number’s 0977 023 685.
Salesman: Thank you. Right, then. Our packers will arrive on the morning of the 19th. I’m afraid I can’t give you an exact time, but it should be around 10 o’clock, depending on the traffic.
Customer: Thank you. As long as they finish before 1 o’clock, I don’t mind.
Salesman: I’ll make a note of that. Right, I’ll just deal with the payment...