Using filters to allow preset and user-defined donations

I see from this post that it's not curently possible to have a choice of preset and user defined donation amounts out of the box, so I thought maybe I could do it by hooks.

I've got a multiple choice field 'Donation' (input_1) with £10, £20, £50 and 'other', and I'm using conditional logic to show the 'Your own amount' field (input_2) if 'other' is chosen.

The problem is the PayPal screen always shows the name of both fields as the item name, which is very confusing. E.g. "Donation, Your own amount" or "£10, Your own amount".

I thought I could use the gform_pre_submission hook to unset input_1 if it is set to 'other', or to unset input_2 otherwise, but this isn't working:

add_filter( 'gform_pre_submission_3', 'only_one_donation_field' );

function only_one_donation_field( $form ) {

	if ( isset( $_POST['input_1'] ) and 'other' == $_POST['input_1'] ) {
		unset( $_POST['input_1'] );
	} else {
		unset( $_POST['input_2'] );
	}

	return $form;

}

Am I doing something wrong or is this just not possible?

Many thanks

Is it possible to use this setup with the main Donation Field as required? It doesn't seem to work. After selecting other, the field is essentially blank on submit and so even though the second user defined field should populate this field, it doesn't or at least doesn't before the validation check.

Is there a way around that?

Thanks,
Jonah

Gotcha, thanks Carl!