Thank you all for your replies! I really appreciate the time you all took to help me.
I've just come home for the day, and one of the things I did was go to another shop that sells backpacks. The salesperson there was knowledgeable, but kept on pushing one particular backpack that sounded good (both front loading and top-loading, and it looked like it had good support), but it was 85L and 3kgs when empty! :shock: He assured me that it would be fine for me, an 18 year old female, to take hiking for 800km. I'm glad I started this forum so I can see more opinions (I'd already read a couple of other backpack-related forum posts before posting this one) that I really shouldn't do that.
Haha, I definitely agree with you, whariwarirangi... I always end up packing more when I have more space. It's easy to get into the "I should bring this just in case!" mindset.
Thanks susannafromsweden for your advice about the sandals and the information about your Osprey packs. It sounds like a good brand, I'll check them out.
Shipping ahead my main backpack and carrying a smaller 40-45L backpack seems like a very good option, so thank you to everybody who suggested those.
Sending my pack ahead to my accommodation every night doesn't seem practical for me since I'd like to maintain flexibility about exactly where I stop, and I'm on a budget and (I think) that would work out more expensive than shipping my excess stuff ahead. But thanks for the idea jastrace, and I'll start thinking of alternative ways to approach my problem.