Upsell's new plans are usage-based. The monthly price is adjusted after 30 days based on your monthly offer views or your store’s monthly order count, whichever is greater.
Because your Shopify billing cycle is different from Upsell’s billing cycle, the app fee may be prorated to match your Shopify billing cycle. This can happen for both the initial charge and if you move to a new plan tier.
For example, if you begin a 30-day Shopify billing cycle on a $9.99 Upsell plan, and move to a $24.99 plan in Upsell on day 15 of your Shopify billing cycle, then you're charged $9.99 + ($24.99 - $9.99) * (15/30) = $17.49 USD.
If you install Upsell or move to a new plan tier in the middle of a Shopify billing cycle, you will see a prorated charge.
For more information about Upsell's pricing, please visit Bold Upsell Pricing & Overview.