1. Be sure with your size. Sizes vary from brand so make sure you know what is your size for the brand you are buying. most of shopping websites have table of sizes with measurements. so grab a meter and check your sizes. if you're buying dresses some websites also say what is the length of the dress which is important you don't want to buy a maxi dress and its midi. this also apply for the shoe, make sure you know what is the size of the heel. 
2. if you're still not sure about the size, go to Instagram and search the brand on tags nowadays you can always find someone with the thing you want to buy. 
3. Payments. most of stores use the visa system or paypal. Here in UAE there is something that i love which is Cash on delivery. usually if you choose this method you pay a small amount but i think its great for those who don't have a credit card. you just place your order and some days later its at your door and that's when you pay. 
4. make sure there is a return policy or you will get stuck with something that doesn't fit.
5. Look for online coupons. a lot of this online shopping websites associate with bloggers that use their clothes and have some coupons so search them on google. you can usually get 10% or 20% off

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