How do I find out my Amazon Merchant Account ID

Hi

Lydia from Handmade Support here,

just adding to Shrewsbury_Marine_S1 info there;
it is Merchant Token that is available in your seller central > settings > account info > merchant token:

https://sellercentral-europe.amazon.com/sw/AccountInfo/MerchantToken/step/MerchantToken?ref_=macs_aimertok_cont_acinfohm

If you need to clarify what you are looking for/why, it is best to raise a case via the security of your seller central:
You can contact seller support through your seller central > help > contact us link:
https://sellercentral-europe.amazon.com/cu/contact-us?encoding=UTF8&ref=ag_contactus_shel_xx