The right way would be using OrderValidationService, which will validate the order for any issues (like out of stock items, price changed etc.), and apply promotions 

For the record, I don't recognize ICartPipeline, nor ICartService. They are likely part of your solution, not a part of the platform APIs

Thanks Quan for your response, yes those methods are part of our solution.
Also I’ve refined a safe and repeatable approach to force a full order recalculation before SaveAsPurchaseOrder in Commerce 14. This ensures capture the latest prices, promotions, taxes, and validations, mitigating edge issues like inventory changes or price updates mid-checkout.

Recommended Full Recalculation + Order Save Flow

Use a helper routine that runs this sequence on the cart - then converts to a PurchaseOrder only after everything is fresh:

// Example helper method
public IDictionary<ILineItem, ValidationIssue> RecalculateCart(ICart cart)
{
    var issues = new Dictionary<ILineItem, ValidationIssue>();
    var contact = _customerContext.CurrentContact;

    // 1. Validate items — availability, market, dates, etc.
    cart.ValidateOrRemoveLineItems(
        (item, issue) => issues[item] = issue,
        _lineItemValidator);

    // 2. (Re-)Apply placed prices — catalog or custom pricing
    cart.UpdatePlacedPriceOrRemoveLineItems(
        contact,
        (item, issue) => issues[item] = issue,
        _placedPriceProcessor);

    // 3. Run promotions / discounts
    cart.ApplyDiscounts(_promotionEngine, new PromotionEngineSettings());

    // 4. Recalculate totals: subtotal, discounts, tax, shipping, grand total
    _orderGroupCalculator.GetOrderGroupTotals(cart);

    return issues;
}

Checkout / Order Save sequence:

var issues = RecalculateCart(cart);
if (issues.Any())
{
    // Inform user / handle invalid items or price issues
    // e.g. “Cart updated — please review before checkout.”
    return;
}

var purchaseOrder = _orderRepository.SaveAsPurchaseOrder(cart);
_orderRepository.Save(purchaseOrder);

Thanks,

That is basically what OrderValidationService does :) 

yes - good point!
OrderValidationService does wrap a similar sequence internally (validation → price processors → promotions → totals), which is why my custom flow looks very close to what that service does under the hood.

In our case, we needed a little more control over when recalculation happens (specifically right before SaveAsPurchaseOrder) and the ability to plug in some project-specific logic around:

• custom price sources

• extra line-item validations

• address-dependent recalculation

• enforcing a “cart state must not change after this point” rule

So the custom recalculation pipeline is effectively the same pattern as OrderValidationService, just with more explicit hooks for our checkout flow.

Thanks for pointing it out - definitely helpful for others reading the thread!