I joined Amazon Prime very soon after it was offered in Australia (a couple of years) ago, and find it a compelling service offering.
There are some wonderful TV series and movies on Prime TV, Kindle Unlimited is an excellent source of e-books, and Amazon customer service is pretty good (for example, easy returns of unsatisfactory/faulty goods), all which lead me to remain a happy customer.
When choosing and ordering items, there are a few things to consider, which I've tried to demonstrate via three screenshots, which are for my account on Amazon Prime AU (Australia), but should be applicable to other Amazon country websites.
(1) Amazon Prime membership should offer you fast delivery
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(2) Seek out the best purchase/delivery options
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(3) Choose items carefully
DO YOUR RESEARCH … Amazon items can usually be found at great prices, but not always the best. Be sure to compare with other online sources, such as eBay.
Exactly the same item from different Amazon AU web pages (that is, different sellers) will usually be priced differently.
Once you've decided on a particular item, be sure to search for the identical item on different Amazon web pages.
Sometimes it's a matter of only a few cents or a few dollars. but occasionally the price difference can be huge, below being one glaring example: