ApplePay on iPad

At the core of this is a simple inescapable fact; an iPad is not a “big” iPhone - each is not a substitute for the other. Each device has hardware and software features not present on the other.

Your iPhone can be used to make “contactless payments” at stores POS (Point of Sale) terminals - because your iPhone supports NFC (Near Field Communication), this being special hardware that is used to communicate with other NFC-enabled devices (such as POS).

By contrast, your iPad does not have NFC hardware capability - and cannot communicate with a POS terminals or other NFC hardware. As such, unlike your iPhone, iPad can only use ApplePay for online payments.

I hope this information is helpful in clarifying some of the fundamental differences between iPhone and iPad - and Fully answers your questions.

Here it says on an ipad Apple Pay is only valid for online, within apps, person to person or in business chat, so it doesn’t work in stores

Apple Pay is compatible with these devices - Apple Support

They are probably operating under the presumption that someone is not taking their ipad to the grocery store