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I've heard from people doing paypal with shopify that shopify can hold the money (in some cases) up to 90 days, n we're talking like thousands of dollars here.
How to avoid this from happening to me if I start my shopify (ecommerce) business n use paypal too?
It can't be avoided, sorry. You have to slowly, steadily, gradually build a good standing with your account and develop best practices as a seller (decent handling and ship time, provide tracking numbers, good customer service) to avoid disputes and customer complaints. Not some overnight, flood of money gold rush.
Also read up on seller protection program (click "Legal" to access the Seller protection program terms in the PayPal User Agreement) to see if your business model is a good fit with PayPal because not all transactions are eligible. For drop shipping to work, you need to have a good relationship with your drop shipper and they have to have good quality merchandise but if you are selling cheap low quality stuff, then make sure it is as described so customers aren't surprised and disappointed and file disputes and request returns.
If you're a new seller on PayPal, funds is subject to PayPal's 21 day funds availability hold but there are tasks you can do to receive the money sooner.
ok lets say I'm planning to grow big with this dropshipping stuff n using paypal
I'm sorry that there is a likelihood that you will face delayed funds availability. In addition to the new seller holds that @sharpiemarker describes, there is a possibility that, if there is explosive growth, there may need to be reserves instead. The risk associated with dropshipping is much higher, and reserves may be a necessity. I'm afraid that it is not possible to determine initially which will be in effect.
I'm sorry for the inconvenience.