Maxim UU

В общем сделал так и заработало:

интерфейс

public interface APIv1 {

    @Multipart
    @POST("/api/v1/order/create")
    Call<ResponseBody> SendOrder(@PartMap Map<String, RequestBody> params);

}

упаковка параметров:

RequestBody rb;
        LinkedHashMap<String, RequestBody> mp= new LinkedHashMap<>();

        rb = RequestBody.create(MediaType.parse("text/plain"), String.valueOf((int)OrderProducts.getBoxSumm()));
        mp.put("price", rb);

        rb = RequestBody.create(MediaType.parse("text/plain"), "1");
        mp.put("number_person", rb);

        rb = RequestBody.create(MediaType.parse("text/plain"), "cash");
        mp.put("payment_type", rb);

        rb = RequestBody.create(MediaType.parse("text/plain"), "yes");
        mp.put("delivery", rb);

        rb = RequestBody.create(MediaType.parse("text/plain"), this.clientname);
        mp.put("client[name]", rb);

        rb = RequestBody.create(MediaType.parse("text/plain"), this.clientphone);
        mp.put("client[phone]", rb);

        rb = RequestBody.create(MediaType.parse("text/plain"), this.clientaddress);
        mp.put("client[address]", rb);

        List<OrderProducts> orderProducts = OrderProducts.getOrderProductsNew();
        for (int i = 0; i < orderProducts.size(); i++) {
            OrderProducts p = orderProducts.get(i);

            Product product = Product.getProductById(p.productid);
            if (product != null) {

                rb = RequestBody.create(MediaType.parse("text/plain"), String.valueOf(product.extid));
                mp.put("products["+ String.valueOf(i)+"][product_id]", rb);

                rb = RequestBody.create(MediaType.parse("text/plain"), String.valueOf(p.amount));
                mp.put("products["+ String.valueOf(i)+"][amount]", rb);

            }
        }

выполнение запроса:

Call<ResponseBody> call = intface.SendOrder(mp);

        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call, retrofit2.Response<ResponseBody> response) {
                Log.v("Upload", "success");
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
                Log.e("Upload error:", t.getMessage());
            }
        });