Give new customers a free trial when purchasing a subscription.

rwstripe-trials.php

<?php

/**
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 */

// Copy from below this line

/**
 * Add a 7 day trial when sending new customers to checkout.
 */
function rwstripetrial_checkout_session_params( $checkout_session_params, $price, $customer_id ) {
	// Only apply trials to subscriptions.
	if ( $checkout_session_params['mode'] === 'subscription' ) {
		// Only allow a trial if the customer has not already had an invoice.
		$invoices = Stripe\Invoice::all( array( 'customer' => $customer_id, 'limit' => 1 ) );
		if ( empty( $invoices['data'] ) ) {
			$checkout_session_params['subscription_data']['trial_period_days'] = 7;
			$checkout_session_params['payment_method_collection'] = 'always'; // Delete this line if you want users to be able to trial without entering payment method info.
		}
	}
	return $checkout_session_params;
}
add_filter( 'rwstripe_checkout_session_params', 'rwstripetrial_checkout_session_params', 10, 3 );

/**
 * Show a 7 day trial in product price.
 */
function rwstripetrial_format_price( $price_text, $price ) {
	// Make sure this price is recurring and not a "choose your own price".
	if ( empty( $price->custom_unit_amount ) && $price->recurring ) {
		// Set up a default currency format.
		$currency_format = array(
			'decimals' => '2',
			'thousands_separator' => ',',
			'decimal_separator' => '.',
			'symbol' => '&#36;',
			'position' => 'left',
		);

		// If Stripe Price has a different currency, check if we have a format for it.
		if ( ! empty( $rwstripe_currency_variations[ strtoupper( $price->currency ) ] ) ) {
			// We have a varaition for this currency. Merge it with the default.
			$currency_format = array_merge( $currency_format, $rwstripe_currency_variations[ strtoupper( $price->currency ) ] );
		}

		$format_number = number_format(
            (float) $price->unit_amount / (float) pow( 10, $currency_format['decimals'] ),
            $currency_format['decimals'],
            $currency_format['decimal_separator'],
            $currency_format['thousands_separator']
        );

		ob_start();
		?>
		<div style="text-align: center;">
			<p>
				<span class="rwstripe-price-unit">7 days free</span>
			</p>
			<p>
				<span class="rwstripe-price-per">then $<?php echo preg_replace( '/' . preg_quote( $currency_format['decimal_separator'], '/' ) . '0+$/', '', $format_number ); ?> per <?php if ( intval( $price->recurring->interval_count ) > 1 ) { echo  $price->recurring->interval_count  . ' '; } echo $price->recurring->interval; ?></span>
			</p>
		</div>
			
		<?php
		$price_text = ob_get_clean();
	}
	return $price_text;
}
add_filter( 'rwstripe_format_price', 'rwstripetrial_format_price', 10, 2 );

// If you need more control over how the "purchase" box displays, you can use the `rwstripe_restricted_content_message` filter.