If it is well written, the terms of your shop lease agreement should cover (admittedly from within a large company that most landlords are desperate to have as a tenant);
a) what fabric maintenance you are responsible for (usually but not always includes windows, doors and structural repair as a result of any fittings/signs you install – however many now also include general fabric of the building clauses as well – obviously you would have had the fabric of the building surveyed and previous works accounted for to ensure that it is well maintained before signing the lease agreement if this is the case to ensure you are not picking up previous tenants liabilities)
b) most of the commercial lease agreements I have encountered , contain payment terms (and timings) including agreed billing cycles/extraordinary costs and a mutual notification of works process (e.g. getting permission to hang a new sign requiring drillings, getting informed that a roof repair is taking place)
In essence, if your tenancy agreement is loosely worded, the landlord can probably get away with any shyster practices he chooses, however did the agreement out and have a good read before paying the bill