The price at the time you place your 1st S&S order for an item is locked in/guaranteed for the first shipment only. If the price of the item increases before shipment, the higher price will show on your S&S summary page but you will not be charged the higher price on the first delivery. Your S&S summary page also does not reflect any coupons clipped or promo credits applied, even though they will be deducted from your order. In other words, no matter what is in the summary page, you will ONLY be charged whatever your invoice and order confirmation email says (even if the price fell significantly, so cancel and reorder at the cheaper price in that case).
Every month AFTER your first order, it is your responsibility to check the price of the items in your subsequent monthly deliveries and decide if you want to skip delivery entirely because the price went up or cancel/skip and then place a new order to get a lower price.
Fortunately, Amazon sends you an email a few days before the deadline to make changes to your monthly order that confirms what you will be charged for each item, including alerting you to any price increases. By the time you receive that email, an invoice has been created so those prices are locked in and you’ll know EXACTLY what you will be charged… so you can decide to let the order stand, skip a shipment, or cancel/skip and reorder. No invoice = no price guarantee.
When I get my email every month, I open the product page for every item to look for price drops and any new coupons. I also compare the current invoice price/product price with what I paid on previous orders (search your order history to pull up all previous invoices). If I can get a better deal due to a lower price and/or coupon, I cancel/skip and reorder. If there is a significant price increase compared to what I paid in the past, I skip that month’s delivery and hope for a lower price the next month. I also add ALL of my S&S items to my cart using the “one-time purchase” option and then move them from my cart to the save later section of my cart. This allows me to easily check for significant price drops for all of my S&S simultaneously simply by clicking on my cart periodically throughout the month (ETA: new coupons available will also be displayed in the cart/saved for later, but you have to scroll to see them).
I also never do S&S deliveries with less than 5 items to ensure that I get 15% off everything. I keep a list of several really cheap S&S items that I actually want on reserve so that I can tack one or more onto a delivery to reach the 5 item minimum to get the 15% discount.
HTH