Is there an "Amazon" equivalent in SE Asia?

There are bunches of them. In Vietnam and I think in Cambodia now is Lazada. If you want a rebranded amazon experience you could try ubuy. Others you can find via google. Ubuy lets you buy from the US amazon store then charges you the cost of the product, shipping and incoming customs duty. Shipping here I would never use Cambodia post if its something you want to receive. I use DHL and fedex. They will call your cambodian phone number when attempting delivery. In Siem Reap, our street has no name and number BTW. I have shipped before to a Khmer friend that manages a guest house. Its downtown and known. Ubuy ships with either of the services above. I also have had things shipped to cambodia post in the past. One thing sat in the Phnom Penh central post office for months and I finally gave up hope but I traced it and it had landed in the central post office. I was out of the country then so I had a Khmer friend go get it. They needed copies of my passport, some numbers, etc.

That's just my experience. Yours may vary.