was the phone Brand New when you got it ?
To start receiving junk SMS soon after setting up, implies the phone may have:
- been pre-owned and NOT cleared correctly
- or the Phone Number has be re-used by the provider
- or the company that sold you the phone is unscrupulous and sold the number (possibly through the darkweb) to a SPAMMING Agent hence the amount of junk and nuisance calls/sms.
Are the Paypal SMS's definitely from PAYPAL they could be from a different number made to look like Paypal - if the transaction has been successful and you already receiving the ordered items then there is no reason why Paypal would need to send you an SMS to confirm the order ? If the messages are genuine from Paypal then they must have the number in their system that you say they don't, you should be able to check this by logging in to Paypal and viewing your account profile and settings.
If a Scammer has the correct name and details of your order then they must be getting this info from somewhere and also know your phone number cannot be random coincidence if they have seller details and transaction correct as well, they could do this with a Trojan type virus on either the machine you used to place the order or even unknowingly on the sellers system. Is it the sellers name and number that you refer to as being well known scammer when you googled ?
Have you checked your systems for Malware and Virus's ?
I recommend you try Malwarebytes (Free or Paid versions) on your laptop/pc to check for unwanted programs and other Malware.