Question about Japanese Auction Sites

And lastly about shipping. If I were to buy a few different items from different sellers, the shipping agent site that does this, will I be charged for shipping per item or will it be combined all together? 

I've used Jauce a few times for hard to find rare items. It's complicated and expensive so as the others say, it's a last resort for me.

If I recall you have up to 30 days from when you purchase something to when you can get them to ship it to you so as you mention you can accumulate multiple purchases and combine them. Don't forget though, you pay the internal shipping within Japan for every item and then pay the shipping from Jauce warehouse to your final destination in the US.

Also, you must have enough credit in your Jauce account, pre-loaded, so you can bid on an item. But if the final shipping turns out to be more than you had pre-loaded you can add funds to make up the shortfall after winning an auction.

I always under estimate how much something will cost because of the currency conversion and conversion fees. Here's a breakdown of a purchase I made a while ago - the initial bid price was acceptable and I was happy to spend about $100 bucks but you will see the fees and shipping then loaded it up:

Initial Bid: 7,000¥
Amount saved: 850¥ (I won for a lower bid so got some yen back)
Fee for winning auction: 892¥
Local delivery charges: 810¥
Bank charges for auction: 300¥
International shipping & packing: 3,520¥

So my total transaction cost to me is 11,672¥ on an initial 7,000¥ bid. That's an extra AUD$70 bucks you need to allow for.

They actually have a service fee estimator and a delivery fee estimator. Check out both here:
Service fee Estimator
https://www.jauce.com/fee_check.php
Delivery Fee Estimator
https://www.jauce.com/price_check.php

Also don't forget to check your card provider because for sure they will slug you with foreign conversion fees.

Good luck!