Add shipping-related filters to avoid MISSING_%field_name% errors

[strangerkir]

Ticket: PCP-4220, also PCP-1546 and PCP-952.

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Description

Two filters added:
woocommerce_paypal_payments_shipping_preference and woocommerce_paypal_payments_shipping_preference.

There was a problem with disabled shipping fields. If in any way some required shipping field is disabled, PayPal returned an error (e.g. MISSING_SHIPPING_ADDRESS or MISSING SHIPPING_CITY). This happened because shipping_preference was set to SET_PROVIDED_ADDRESS, which made PayPal expect shipping fields.

It was decided not to make changes in how the plugin behaves, but to add filters to let merchants deal with this problem if they manipulate checkout fields.

Example code to fix MISSING_* error:

//Disable PayPal expectaion for shipping fields.
add_filter('woocommerce_paypal_payments_shipping_preference', fn() => 'NO_SHIPPING');

//Prevent sending shipping fields from order and cart
add_filter('woocommerce_paypal_payments_shipping_needed', '__return_false');

I'm attaching the same code as a plugin.
paypal_disable_shipping.zip

Steps to Test

1. Install and activate the Flexible Checkout Fields plugin.
2. Go to the Flexible Checkout Fields plugin settings, select the Shipping sub tab and disable Street Address field (see screenshot).
3. Make sure shipping is enabled.
4. Go to the classic checkout with any physical product in cart and complete checkout using Pay with PayPal button.
5. Observe an error MISSING_SHIPPING_ADDRESS.
6. Install and activate plugin attached to this PR.
7. Go to the classic checkout and try to pay again, the error shouldn't happen anymore.

To merchants, who has this error we might recommend the snippet above, but not the plugin. We would need to add author and other details before providing it.

Documentation

• This PR needs documentation (has the "Documentation" label).

Changelog Entry

Added filters for fixing MISSING_%field_name% checkout error when some shipping fields are missing.

[Narek13]

LGTM. Thanks