Keep one reliable source of facts
Before writing either message, note the product name, material, colour, sizes, price if confirmed and how someone can order. If delivery or availability depends on location, say so instead of making a blanket promise.
This small fact sheet saves you from writing three versions that disagree with each other.
Make the WhatsApp message easy to reply to
People read WhatsApp quickly. Lead with the product, share the two or three details that matter most, and end with a simple action such as asking for a size or sending a message to order.
A sample structure is: product name, key details, available options, then the next step. Keep long specifications for the listing or a follow-up answer.
Give Instagram a clear caption
On Instagram, the image does some of the work. Use the caption to add context the image cannot show: material, sizes, what is included and how to enquire. Avoid a wall of hashtags or claims you cannot verify.
Read both versions side by side. They should sound different because the channels are different, but they should never contradict each other.