In short: Each item only gets one product discount, so during a 20% sitewide sale a 10% product discount code gives nothing extra. Make the influencer code an Order Discount with free shipping, then allow the sale and the code to combine. The 10% comes off after the sale price.
Why one code: Shopify's own discount codes each hold one kind of discount, so 10% off the order and free shipping would be two codes per influencer. In OneCode: Stackable Discounts it's one code.
Steps
Click Create discount and enter the influencer's code, for example
ANNA10.Click Add Discount Type, set Discount Type to Order Discount and enter
10in Percentage.Click Add Discount Type, set Discount Type to Shipping Discount and keep Free Shipping.
In Combinations, keep Product discounts ticked, and Shipping discounts too if your sale includes free shipping.
Click Save discount.
Open your sale discount: in Shopify's Discounts page if it's a Shopify discount, or in OneCode if it's a OneCode discount. Under Combinations, click the pencil icon, tick Order discounts and Shipping discounts, and save. New OneCode discounts already have them ticked.
What the customer gets
A $100 cart during a 20% automatic sale, with ANNA10:
The sale takes 20% off the items: $80.
ANNA10takes 10% off the rest: $8.The customer pays $72, with free shipping.
If your sale uses compare-at prices
If your sale lowers prices with compare-at prices instead of a discount, you don't need an order discount: a Product Discount in the code already comes off the sale price. To change that, use a product discount in the code and click Add sale item rule:
Discount from the compare-at price: the customer gets the code's percentage off the original price, or keeps the sale price if that's lower. The two don't add up. With a 20% code, an item reduced from $100 to $85 ends at $80.
Exclude sale items: the code only discounts full-price items.
See Sale items and subscription items.
Variations
Free shipping only over $50: give the shipping discount the condition Cart Subtotal at least
50.A code for every influencer: save one discount and import codes like
ANNA10andJOE10with Create Bulk Codes, then check Uses to see who's driving sales. See Unique single-use codes for emails, influencers or vouchers.
Good to know
Both discounts have to allow combining. If either one doesn't, the customer can't use them together. See How OneCode discounts combine with your other discounts.
If the sale also gives free shipping, the customer simply gets free shipping. Shopify applies one shipping discount per order from separate discounts.
On Shopify Plus, two product discounts can stack on the same item with Multiple per product.