Customise Font for Shop Product Items Details Title

Use this CSS along with your font family:

.collection-type-products .ProductItem-details h1.ProductItem-details-title, .collection-type-products .ProductItem-details .product-price, .collection-type-products .ProductItem-details .ProductItem-details-excerpt, .collection-type-products .ProductItem-details .variant-option, .collection-type-products .ProductItem-details .product-quantity-input {
  font-family:'YOUR-FONT-FAMILY' !important;
}

Let me know if that helps!