[Beginner] 405 error when trying to post/put elements

Hi there,

I am new to web development and I am currently trying to build an e-commerce website as a project.

I managed to build the front-end using Vue and Vuex to store the shopping cart. I would now like to be able to save the Shopping Cart Items into my database if the user is logged in.

I created a model for it: ShoppingCartItem
-User: One-to-One user_permissions_user
-Variation: One-to-One variation (each product has different variations which have a product reference)

I set the permissions so that authentificated users can post/find/get etc

Right now in Vuex my addToCart action looks like this:

addToCart({ commit }, cartItem) {   
      var found = false;                                                               
      var index;                                                                       
      for(var i = 0; i < this.state.cart.length; i++) {                                 
        if (this.state.cart[i].variation.Reference == cartItem.variation.Reference) {  
            found = true;                                                              
            index = i;                                                                 
      if (found) {                                                                     
        if (user) {
          axios.put('http://localhost:1337/shopping-cart-item', { variation: { Reference: cartItem.variation.Reference }, users_permissions_user : { id: user.user.id }}, { quantity: this.state.cart[index].quantity + 1 }, { headers: authHeader() })
      } else {
        if (user) {
          axios.post('http://localhost:1337/shopping-cart-item', { ...cartItem, user: user.user }, { headers: authHeader() })
          .then(() => commit('addToCart', { ...cartItem, user: user.user }))
        } else {
          commit('addToCart', { ...cartItem, user: 'guest' })            

I changed the update service to look like this:

async update(params, data) {
     const existingEntry = await strapi.query('shopping-cart-item').findOne(params);
     const isDraft = isDraft(existingEntry, strapi.models.shopping-cart-item);
     const validData = await strapi.entityValidator.validateEntityUpdate(
       { isDraft }
     const entry = await strapi.query('shopping-cart-item').update(params, validData);
     return entry;

But I am unsure if params is a customizable object or if it’s a req.params equivalent. I do not have the object id so I removed :id from the route.

I am pretty sure the update feature must have many problems, but as a start I am wondering why the post method doesn’t work.

axios.post('http://localhost:1337/shopping-cart-item', { ...cartItem, user: user.user }, { headers: authHeader() })

The cartItem is built on the frontend and only missing the user info, which I get in Vuex

const user = JSON.parse(localStorage.getItem('user'));

I know a lot of this code needs to be refactored / put into functions but right now I just want to understand how it works before I abstract many things out of it.

At this point I would be okay to pay for mentorship to be able to understand all this a bit better.
Thanks for your help!