Valid & Invalid transactions

Q: There's a trend among large retailers of using a marketing strategy called odd pricing, eg. $4.99, instead of $5. This is based on the psychological trickery of making a price more appealing. Muslim informal traders have now adopted this same strategy.

Firstly: Is this allowed, considering the origin and deceitful nature.

Secondly: These Muslim traders accept the full amount of $5. Unlike the large retailers, who give your change, the Muslim traders offer no change to the customer. This price is sometimes $199, and when accepting $200, no change is offered. Is this permissible? Surely, this is a form of stealing. At the very least, trickery.

When called out, they offer 'Urf as an excuse, saying this is the general usage in a community. However, the retailers who use this strategy gives the customer his due change. So it does not equate to the same.

Q: I was at this fastfood shop last week and I bought a 3 chicken wings meal. I came home and saw that I was given 4. Normally I don’t worry about this since usually I assume this occurs if one of the wing pieces is small so they give another to compensate, however, because I’m dealing with religious OCD and doubts, I’m worried. I called them today and they confirmed that sometimes they do this for the reason I mentioned above. Can I assume that this is what occurred in my case instead of thinking that they made a mistake?

Q: Shein, an online store has a points system. For everyday the user checks in they are given points, for every review on items bought u are given points.

When orders are received and you confirmed its received, you are given points and these points are then accumulated and converted into rands for which you can use for purchases.

Is this points system valid?

Also, they have a coupon system where if you purchase over a certain amount, there is a discount. Is this permissible?

Q: Are we as Muslims permitted to buy from Shein, the online store?

Q: Zaid buys a car with khiyaarus shart. Can he sell it off within the 3 day period, make his profit, and then pay the due amount to the seller?

Q: Is it permissible to buy a product from someone if we know that he bought that product by means of an interest based loan?

Now he has finished all installments and possesses the product and is willing to sell it off..

Q: I bought an electronic item from a store and it was discussed that the amount would be 67000 INR even if I pay once or in 3 installments. On the bill it was mentioned that 22333x3 will be deducted from my card every month for 3 months and also it was mentioned as 65300 as principal and 1700 as interest. Is this allowed as the amount was fixed?

Later, I found out in the statement that around 100 rupees was charged extra and total amount which they took was 67100 INR.

Q: There is a site who give 0.35$ every day to it's verified users who login every day. It's like daily login rewards. Verified users are those who complete their verification process through identity card. Is the money earned haraam or halaal?

Q: I have a question regarding interest. I have a financial issue in my business. I need around 5 million to complete my running order. A friend of mine has an electronics business. He is selling his products at a higher price in easy monthly installments. As I told my problem to him, he bought a vehicle for 5 million pkr and sold me this vehicle for 7.5 million in six monthly installments. I sold this vehicle to the same owner at a lower price (4.99 million) and took money for completing my order. Is this haram in Islam? Because my friend is taking money against a product, not against money, and I'm paying him for a vehicle, not for money. If it is haraam, then what is the solution because I already used the money and invested in my business after selling the vehicle. 

Q: I often sell goods on lay-by. It is becoming more frequent that the buyer cancels the lay-by after having paid part of the agreed amount. I then refund him the paid amount.

When this happens, is it permissible for me to charge the buyer a handling fee or admin fee of say 10%?