Nothing breaks and nothing stops. Going past your monthly allowance does not lock the app, drop orders on the floor or leave a purchase order sitting unread in an inbox, which would be the one failure mode that actually costs you money.
What happens instead is a conversation about the right plan. If you are consistently over, the next plan up is almost always cheaper per order than staying where you are, which is why the per order price falls from $3.33 on Pay as You Go to $3.03 on Starter to $2.50 on Growth. Paying annually takes Starter to $2.42 and Growth to $2.00. Moving up mid-cycle is a plan change in Shopify, not a renegotiation.
If your volume is seasonal rather than growing, say a heavy autumn buy followed by a quiet winter, Pay as You Go exists for exactly that shape. It is a single $50 charge for 15 successful orders that stay valid for 12 months, so you are not carrying a monthly subscription through the quiet half of the year.
Above 100 orders a month there is no published price, and we quote instead. At that volume the interesting questions are usually about integrations, service levels and how many order channels you want consolidated, not about the per order price.