Online shopping in the USA

Amazon Prime has a small member fee but generally deliveries are next day and free returns.
Most retailers offer delivery. Some charge, some do not charge w/store credit cards and/or minimum purchase. Some offer free delivery and returns with store pick up.
Sales tax is generally applicable when a retailer has a brick and mortar location in state or the state requires it. At up to 10% it can add up.
International can turn into major headaches with customs or even returns.

I started shopping online 20+ years ago.  In the beginning, I had to return the merchandises quite often, but eventually, I've found some specific companies whose products were perfect for me.  In the last decade, I've kept everything I ordered.

Amazon is the largest supplier of my electronic, e-books, and miscellaneous items, but never of clothing.  I'm allergic to synthetic fabrics, I have sensitive facial skin, I'm picky of what I put on my face/body, and I don't wear anything made in China.  That combination made me a difficult customer, but I'm also a loyal customer, that's why I've stayed with the same companies for decades.

Clothing:  Mango (Spain), Pure Collection (England), American Giant (US), Haris Cotton (Greece), Uniqlo (global), The Wool Company (New Zealand)

Shoes:  TOMS (US)

Outdoors gears:  66° North (Iceland), Hestra (Sweden)

Skin care products:  Mon Coin Santé (France)

Tea:  Mariage Frères (France), Dilma (Sri Lanka)

Essential oils:  Edens Garden (US), Plant Therapy (US)

Photographic equipment:  B&H (US), Adorama (US)

I only pay shipping to Mon Coin Santé and Mariage Frères, but I've always ordered enough goods to take advantage of the rate/weight.  With everyone else, shipping is either free or free over minimum order. 

Sales tax is either free or insignificant from US companies.  From overseas, no custom paid when value of goods is less than a certain amount. 

Delivery time is between good to excellent -- next day with Amazon, about one week from overseas (only 3 - 4 days from France with DHL.) 

Customer service from all of those companies is excellent.  Aside from skin care products and tea (hygienic reason), all of them offer free return.  Some of the clothing companies have 12-month return policy. 

High quality goods, comparable prices, utmost convenience, and time saving made me an almost exclusively online shopper, and a very satisfied one at that.

I've been back in the States since August, and have ordered everything I need to take to VN (our residence at the moment). 

From Amazon:  JOBY GorillaPod for camera, ASUS ZenBook with Brinch carrying bag, Bose Soundlink with co2CREA carrying case, Panasonic earphones, Tarriss luggage scale, Bürstenhaus Redecker bath brush, Sonicare replacement toothbrush heads, RFiD credit card holder, dry brushes for facial and body from a local beauty supplier, and Darphin skincare.

From Mariage Frères and Mon Coin Santé:  tea and skincare

All of those online orders just in the last two months alone, received the last one this afternoon, and not a thing was wrong with any of them. 

Yessiree, I'm a happy online shopper. 

Re: your question "Are there particular products that are cheaper to buy online or that are perhaps not available in the USA?"  The skincare I use and tea I drink are either not available or available but with fewer selections, while the prices are much more expensive at stores and online both in the States.  Half of the clothing I wear are not available in the States at all.

You most likely can buy anything online in States.Amazon.com will be your best option.You do have to pay tax depending on the State, you are in.With $50 or more, the shipping is free.You also can open a account with amazon.com and get discount on many things.There are hundreds of website in US, sales all different kinds of goods. Please google the items that you want and chose the online website, that sales the items of your dimind....

With Amazon Prime, everything is 2-day (or next day if you live within 100 miles or so of a warehouse) free shipping.  Without Prime, free shipping kicks in at $35, not $50.

start off with Amazon, then try Google shopping to compare prices, or there are other big retailer that begins to get traction as well like Walmart, Target, Jet, Coscto. Depending on certain categories there are specialize stores too