Q 1 Corinne
First, I am not an American but want to sell [on] Amazon.com
I have sent a few small packages to Amazon FBA.
There was no issue at all until I started sending 15 cartons.
When I sent this 15 cartons, I don't have Federal tax ID number.
Thereby I needed to spend US$ 500 to have freight forwarder to help me.
Then I tried 8 cartons through DHL which declared $1200 for the customs. However, it is still got rejected by the customs.
It seems FEIN is required if i want to ship my inventory to FBA.
I am not trying to escape any tax issues, but to get a FEIN number, I would need a legal address in US. I am not in US.
How do you guys deal with this?
A freight forwarder isn’t the same as a Customs Broker in the USA. Some companies do both, like Western Overseas Corporation. But it sounds like what you need is a Customs Broker.
You shouldn’t need a US address to get an EIN as a foreign entity (person or company). But if you need one (you do need a returns address for amazon or should at least have one), google. I used myaddressus.com - pretty cheap.
If you send in goods over the value of $2500, it’s a formal import so you’ll need a customs bond etc. At that point, I would use a Customs Broker, at least for the first time. That's not the case here, but worth flagging up for future reference. 
Q2 Ben
Here is my newbie experience post #3. So I’ve been selling for about 3 weeks in the UK. Where am I?
I picked a great product. It’s flying off the cyber-shelves. I am about ½ way up page 1 for all my main keywords. I told myself to be ‘happy’ with 5 units a day. I was averaging 10-12 units per day, but have increased my price and now average 7-8 units per day. I have had days of 10+ including a day of 16 units. These are not giveaways, all giveaways were done in the first week. So why did I increase the price?
This first ‘test run’ was 500 units. At the current rate, I’m going to run out quickly. So I’m trying to find the balance between maintaining sales, and not running out of inventory. Am still undercutting some competitors at the current price, but also more expensive than some others.
I believe the reasons it’s going well so far are:
1. Branding. My brand is easily one of the coolest, and as I expand I’ll grow as a brand, rather than as “Bob’s generic stuff” which several of my competitors are doing
2. Social media. Many ‘gurus’ say – avoid social media until you are well established on Amazon. This is not a good move, in my opinion. I have an active twitter and Instagram account, and a new facebook page. OK I haven’t got many followers, but t’s growing and I’m getting a bit of engagement.
3. Branding. I said it again because it’s so important. Branding is everything. Cool brands get bought. Generic crap doesn’t.
Comments
Alex  You are doing well, but don't talk about brand building. People don't care. You drive them in your page and they just see information, reviews and price. Just set Ppc automatic. If doesn't work move on.
Suzi I love to hear that you are growing a strong brand, and that you understand and appreciate how important it is. I cringe so hard when I hear people say your branding doesn't matter...nothing can be further from the truth. Have you had any issues with counterfeits hopping on your listing(s)?
My response: it depends! 
If you just want to make sales on Amazon short-term (say next 6 months), I think it's true that brand doesn't matter much. Initially, customers will not have heard of your brand yet, and they mostly go with good images and price. Also, it is not realistic to expect Amazon to cross-sell your products even if they are in the same niche.
However, if you want to create a defensible business to later sell, you definitely need to create a brand. Also, even if you're never planning to sell, if you want to create a Shopify store, to diversify and lower the risk of Amazon controlling your business,  you will need to develop a focussed suite of products.